Optical recording medium having two separate recording layers

ABSTRACT

In an optical information recording medium having at least two information layer, guide grooves for tracking or sample pits or information pits corresponding to information signals are formed on a surface of a first substrate. A first information layer formed by a thin film for reflecting a portion of a light beam made incident on the first substrate and permitting penetration of a portion of the light beam is formed on a surface of the first substrate. Guide grooves for tracking or information pits corresponding to information signals are formed on a surface of a second substrate. A second information layer having a reflectance higher than that of the first information layer is formed on a surface of the second substrate. Between the first information layer and the second information layer, there is formed a transparent separation layer for positioning the first information layer and the second information layer to be spaced a predetermined distance apart from each other.

FIELD OF THE INVENTION

The present invention relates to an optical information recording mediumcapable of reproducing information signals or recording and reproducinginformation signals by using light beams, a manufacturing methodtherefore, a manufacturing apparatus therefore and an opticalinformation recording and reproducing apparatus making use thereof, andmore particularly to an optical information recording medium having amultilayer structure including a plurality of information layers, amanufacturing method therefore, a manufacturing apparatus therefore andan optical information recording and reproducing apparatus making usethereof.

DESCRIPTION OF THE PRIOR ART

Hitherto, an optical information recording medium has been known whichis capable of optically recording information signals or reproducingrecorded information signals and which is formed into an optical disk,an optical card or the like. In general, the foregoing recording mediumuses a semiconductor laser unit as a light source. By irradiating therecording medium with light beams finely converged by a lens,information signals can be recorded on the recording medium in a largequantity, and information signals recorded on the recording medium canbe reproduced.

At present, investigations have been performed to further enlarge therecording capacity of the recording medium of the foregoing type. Toraise the recording density, it is effective to improve the reproducingresolution by finely limiting the light beam. In order to achieve this,investigations have been performed such that the wavelength of the lightbeam is shortened or the numerical aperture (NA) is enlarged. Moreover,reproducing methods have been investigated in which the focusing ortracking accuracy is improved and crosstalk between signals is preventedso as to allow the recording surface density to be raised.

Although employment of the foregoing methods enables the recordingcapacity per unit area to be somewhat enlarged, a structure having onlya single information layer for recording information encounters alimitation when the recording density is intended to be raised.

If a plurality of information layers for recording information areprovided, it can be expected that the recording capacity can bemultiplied. A method for manufacturing an optical disk having themultilayer structure has been disclosed in, for example, U.S. Pat. No.5,126,996.

A process for manufacturing the foregoing optical disk will now bedescribed. As shown in FIG. 21(a), a first information layer 212 isformed on the surface of a substrate 211 manufactured by an injectionmolding method or the like and having information pits. Then, as shownin FIG. 21(b), a photosetting resin 214 is applied to the upper surfaceof a master 213 having information pits. Then, as shown in FIG. 21(c),the surface of the first information layer 212 on the substrate 211having the information pits and the surface of the master 213 having theinformation pits are caused to face each other. Then, in a state wherethe substrate 211 is pressed, the photosetting resin 214 is irradiatedwith light from a position on the outside of the master 213. As aresult, the photosetting resin 214 is set so that the photosetting resin214 is adhesive-bonded to the first information layer 212. Then, asshown in FIG. 21(d), the master 213 is removed from the photosettingresin 214. As a result, a resin layer having information pits on thesurface thereof and composed of the photosetting resin 214 can beformed. Then, as shown in FIG. 21(e), a second information layer 215 isformed on the resin layer (made of the photosetting resin 214). Finally,as shown in FIG. 21(f), a protective coating layer 206 is formed on thesecond information layer 215. As a result of the foregoing process, anoptical disk having a double-layer structure can be obtained.

However, when the master 213 is separated from the photosetting resin214 (see FIG. 21(d)), the foregoing conventional manufacturing methodcan involves separation being easily taking place at the interfacebetween the first information layer 212 and the substrate 211 or theresin layer (made of the photosetting resin 214). Thus, there arises aproblem in that the manufacturing yield has been unsatisfactory. Thereason for this can be considered that the adhesivity between the master213 and the resin layer (made of the photosetting resin 214) is made tobe greater than that between the first information layer 212 and thesubstrate 211 or that between the first information layer 212 and theresin layer (made of the photosetting resin 214).

In a case where the substrate 211 is made of a resin, change in theenvironmental temperature or in the humidity sometimes raises a problemin that the manufactured optical disk can be deformed or an error occursin reproducing the signals.

Moreover, an apparatus of a type for reproducing information signalsfrom a plurality of information layers has a problem in that the servooperation becomes unstable due to an influence of light reflected by aninformation layer other than the subject information layer.

SUMMARY OF THE INVENTION

The present invention was found to overcome the foregoing problemsexperienced with the conventional structure, and, therefore, an objectof the present invention is to provide an optical information recordingmedium having a multilayer structure which cannot easily be deformed dueto change in the environment, a manufacturing method therefore, amanufacturing apparatus therefore and an apparatus for opticallyrecording and reproducing information.

To achieve the foregoing object, according to one aspect of the presentinvention, there is provided an optical information recording mediumhaving two different information layers and capable of reproducinginformation signals from the information layers, recording informationsignals on the information layers that can reproduce the recordedinformation signals when irradiated with a light beam, the opticalinformation recording medium comprising: a first substrate having, onone side thereof, one or more selected from the group consisting ofinformation pits corresponding to information signals, guide grooves fortracking and sample pits; a first information layer formed on eithersurface of the first substrate and having a predetermined transmissivityand a predetermined reflectance with respect to the light beam; a secondsubstrate having, on one side thereof, one or more selected from a groupconsisting of information pits corresponding to information signals,guide grooves for tracking and sample pits; a second information layerformed on either surface of the second substrate and having apredetermined reflectance; and a separation layer formed between thefirst information layer and the second information layer that istransparent with respect to the light beam. The structure of the opticalinformation recording medium provides a double-layer recording mediumthat is capable of reproducing information signals from the first andsecond information layers, recording information signals on the firstand second information layers and reproducing recorded informationsignals.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the thickness of thefirst substrate is substantially the same as the thickness of the secondsubstrate. As a result of the preferred structure, a structurevertically symmetrical with respect to the separation layer can beformed. Therefore, even if stress or the like is generated in thesubstrate due to change in the temperature or the like when the opticalinformation recording medium is manufactured, distortion can becompensated. Even if unusual deformation factors act on the twosubstrates due to change in the temperature or the humidity of theenvironment, deformation and warp can be inhibited. As a result, astructure durable against change in the environment can be obtained.Therefore, even if the first and second substrates are made of resin,deformation of the manufactured recording medium causing an error inreproducing a signal can be prevented.

It is preferable that in the optical information recording mediumaccording to the present invention, the below mentioned formula issatisfied;

    R1≈1-A1+(2·R2).sup.-1 -{ 1-A1+(2·R2).sup.-1 !.sup.2 -(1-A1).sup.2 }.sup.0.5

wherein

R1 is a reflectance of the first information layer,

A1 is an absorption ratio of the first information layer, and

R2 is a reflectance of the second information layer with respect to thewavelength of the light beam used to reproduce the information.

It is preferable that the optical information recording medium has astructure such that the reflectance of the first information layer is25% to 40%. As a result of the foregoing preferred structure, arecording medium having the first and second information layers, whichare reproduction only information layers, can be obtained in which theamplitudes of the signals from the first information layer and thosefrom the second information layer are similar to each other and thereproducing amplitude is satisfactorily large.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the secondinformation layer includes a reflecting layer, a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially laminated on thesecond substrate. According to the foregoing preferred structure, asecond information layer, the information of which can be rewritten, canbe obtained.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the first informationlayer has a coefficient of absorption which is substantially zero withrespect to the light beam. According to the foregoing preferredstructure, the quantity of the light that reaches the second informationlayer can be enlarged.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the first informationlayer includes at least two layers of thin films. According to theforegoing preferred structure, the reflectance of the first informationlayer can be enlarged and the quantity of the light that reaches thesecond information layer can be enlarged. In the foregoing case, it ispreferable that the first information layer includes a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially laminated on the firstsubstrate. According to the foregoing preferred structure, a firstinformation layer can be obtained on which information can be recordedor information of which can be rewritten.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that both of the first andsecond substrates have information pits corresponding to informationsignals on either surface. According to the foregoing preferredstructure, a recording medium having a large capacity can be obtained.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the first informationlayer is a reproduction only information layer and the secondinformation layer is a recording and reproducing information layer.According to the foregoing preferred structure, light absorption of thereproduction only first information layer can be set to a low level.Moreover, since the influence of diffraction occurring due toinformation pits can be inhibited, information can be recorded orreproduced with small power.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the thickness of theseparation layer is larger than the focal depth of an optical system forconverging the light beam and smaller than the tolerance of a baseallowed by the optical system. According to the foregoing preferredstructure, information satisfactorily free from crosstalk can bereproduced from another information layer as long as the aberration ofthe light beam to be applied is small.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that information pits orsample pits are formed on the first and second substrates and the shapesof the pits formed on the first substrate are different from the shapesof the pits formed on the second substrate. In the foregoing case, it ispreferable that the width of the pit formed on the second substrate islarger than the width of the pit formed on the first substrate.According to the foregoing preferred structure, diffraction of lightreflected from the first information layer and that from the secondinformation layer can be made to correspond. Therefore, signals can bereproduced stably.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that guide grooves areformed in the surfaces of the first and second substrates and the widthof the guide groove formed in the second substrate is larger than thewidth of the guide groove formed in the first substrate. According tothe foregoing preferred structure, the effect of the guide groovesformed in the surface of the first substrate and the effect of the guidegrooves formed in the surface of the second substrate can be made to bethe same.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that information pits areformed on the surfaces of the first and second substrates and thedensity of the information pits formed on the second substrate per unitarea is lower than the density of the information pits formed on thefirst substrate per unit area. According to the foregoing preferredstructure, information can be reproduced satisfactorily from aninformation layer out of the focal depth of the optical system forconverging the light beam.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that sample pits or guidegrooves are formed in the surfaces of the first and second substratesand the pitch of the sample pits or the guide grooves formed in thesecond substrate is shorter than the pitch of the sample pits or theguide grooves formed in the first substrate. According to the foregoingpreferred structure, a recording medium can be obtained in which signalscan be reproduced satisfactorily from an information layer out of thefocal depth of the optical system for converging the light beam andsignals can be recorded on the same.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that information pits orsample pits are formed on the surfaces of the first and secondsubstrates and the directions of the information pits are opposite toeach other between the first substrate and the second substrate whenviewed from a position upon which the light beam is made incident.According to the foregoing preferred structure, the material and moldingprocess of the substrates can be made to be the same as well as themastering process. As a result, it is only necessary to prepare twotypes of manufacturing apparatuses having the same function or onemanufacturing apparatus can be used commonly when the substrates aremanufactured. Thus, the cost of the facilities for manufacturing thesubstrates can be reduced.

It is preferable that the optical information recording medium accordingto the present invention has a structure such that the information pits,the guide grooves or the sample pits formed on the surfaces of the firstand second substrates are formed into a spiral shape relative to thecentral portions of the first and second substrate and the shape of thespiral on the first substrate and the shape of the spiral on the secondsubstrate are the same when viewed from a position upon which the lightbeam is made incident. According to the foregoing preferred structure,the light beam is moved in one direction from the inner portion to theouter portion or one direction from the outer portion to the innerportion regardless of the information layer having the information pitsto which tracking has been made. In the case where the structure isemployed in which the light beam is moved from the inner portion to theouter portion, a reproduction method may be employed in whichadministration information is detected in the inner portion of any ofthe information layers and an access is made to a required informationregion including the portion between the information layers. Therefore,it can be said that the foregoing structure is suitable for a recordingmedium capable of allowing a high speed access to the informationlayers.

It is preferable that a pair of the optical information recordingmediums according to the present invention and an adhesive-bonding layerare provided and the second substrates of the pair of opticalinformation recording mediums are adhesive-bonded to each other throughthe adhesive-bonding layer. According to the foregoing preferredstructure, a recording medium having a four-layer structure can beobtained in which information can be recorded and reproduced from eachof the information layers by irradiating the two sides with the lightbeams. In the foregoing case, it is preferable that the thicknesses ofthe first substrates of the pair of optical information recordingmediums are substantially the same and the thicknesses of the secondsubstrates of the pair of optical information recording mediums aresubstantially the same.

According to another aspect of the present invention, there is provideda method of manufacturing an optical information recording medium havingtwo different information layers and capable of reproducing informationsignals from the information layers, recording information signals onthe information layers and reproducing the recorded information signalswhen irradiated with a light beam, The method of manufacturing anoptical information recording medium comprises: a first film-formingstep of forming a first information layer having a predeterminedtransmissivity and a predetermined reflectance on a substrate having, onone side thereof, one or more selected from the group consisting ofinformation pits corresponding to information signals, guide grooves fortracking and sample pits; a second film-forming step of forming a secondinformation layer having a predetermined reflectance on a secondsubstrate having, on one side thereof, one or more selected from a groupconsisting of information pits corresponding to information signals,guide grooves for tracking and sample pits; an applying step of applyinga resin layer, which is transparent with respect to the light beam, tothe upper surface of the first information layer or the secondinformation layer; an adhesive-bonding step of causing the firstinformation layer and the second information layer to face each otherand adhesive-bonding the first and second information layers to eachother through the resin layer. According to the structure of the methodof manufacturing the optical information recording medium, the step ofremoving the master is not required. By simply adhesive-bonding to eachother the substrates having the information pits formed previously, arecording medium having a double-layer structure can be obtained. As aresult, the manufacturing yield can be improved.

It is preferable that the method of manufacturing an optical informationrecording medium according to the present invention has a structure suchthat the thickness of the first substrate is substantially the same asthe thickness of the second substrate.

It is preferable that the method of manufacturing an optical informationrecording medium according to the present invention further comprises apressing step of pressing the first and second substrates from theoutsides of the first and second substrates. According to the foregoing,a resin layer satisfactorily free from irregularity in thickness can beformed.

It is preferable that in the method of manufacturing an opticalinformation recording medium according to the present invention aphotosetting resin is used to form the resin layer and the resin layeris irradiated with light from a position on the outside of the firstsubstrate so as to set the resin layer while applying pressure to theoutsides of the first and second substrates. According to the foregoingpreferred structure, a resin layer satisfactorily free from irregularityin thickness can be formed in a short time.

It is preferable that the method of manufacturing an optical informationrecording medium according to the present invention further comprises anapplying step of applying an adhesive-bonding layer, which istransparent with respect to the light beam, to the upper surface ofeither of the second substrates of a pair of optical informationrecording mediums obtained by the method of manufacturing an opticalinformation recording medium according to the present invention; and anadhesive-bonding step of causing the second substrates of the pair ofoptical information recording mediums to face each other andadhesive-bonding the second substrates to each other through the resinlayer. According to the foregoing, a recording medium having afour-layer structure can be obtained by repeating, three times, theadhesive-bonding step for obtaining the recording medium having thedouble-layer structure. That is, the recording medium having thefour-layer structure can be manufactured by a method similar to that formanufacturing the recording medium having the double-layer structure bybasically preparing the same manufacturing apparatuses for the threesteps. In the foregoing case, it is preferable that the thicknesses ofthe first substrates of the pair of optical information recordingmediums are substantially the same and the thicknesses of the secondsubstrates of the pair of optical information recording mediums aresubstantially the same.

According to another aspect of the present invention, there is providedan apparatus for manufacturing an optical information recording mediumcomprising: a first substrate support section for supporting a firstsubstrate; a second substrate support section disposed to face the firstsubstrate support section so as to support a second substrate; anapplying section for applying a resin layer to the upper surface of thefirst substrate or the second substrate; a spacer disposed on a plane onwhich the first substrate support section and the second substratesupport section face each other, the spacer being disposed outward fromthe first and second substrates; and a pressing section for elevatingthe first substrate support section or the second substrate supportsection and pressing the first and second substrates. The apparatus formanufacturing an optical information recording medium allows, arecording medium having a multilayer structure to be producedefficiently, and it can be expected that the recording capacity can beincreased.

It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention furthercomprises a light source disposed opposite to a surface of the firstsubstrate support section that is in contact with the first substrate,wherein the first substrate support section is made of a material thatpermits penetration of a portion of light emitted from the light source.According to the foregoing preferred structure, the photosetting resinlayer is used so that the first substrate and the second substrate areadhesive-bonded to each other in a short time.

It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention has astructure such that each of the first and second substrates has acentral opening in the central portion thereof and concentric or spiralprojection and pit columns or guide grooves on either surface thereof,and a center position correction section for making the central axes ofthe information pit columns or guide grooves of the first substrate andthe second substrate to coincide with each other is further provided forat least one of the first substrate support section and the secondsubstrate support section. According to the foregoing preferredstructure, a recording medium can be obtained in which the deviation ofthe circular arc of the information pit columns or the guide groovesformed on the surfaces of the two information layers can be inhibited.

It is preferable that the apparatus for manufacturing an opticalinformation recording medium according to the present invention furthercomprising first and second shaft sections respectively disposed on thecentral axes of the first and second substrate support sections; andfirst and second inner-portion guide sections each having a taperedportion having an end larger than the central opening of the substrateand another end smaller than the central opening of the substrate, thefirst and second inner-portion guide sections being capable of movingalong the first and second shaft sections. According to the foregoingpreferred structure, the first and second substrates can be secured tothe surfaces of the first and second substrate support sections in thestate where the tapered portions of the first and second inner-portionguide sections respectively are in contact with the central holes of thefirst and second substrates. Therefore, the central axis of the firstinformation layer and that of the first substrate support section can bemade coincide with each other with a value near the limit of mechanicalaccuracy. Moreover, the central axis of the second information layer andthat of the second substrate support section can be made coincide witheach other with a degree near the limit of mechanical accuracy. In theforegoing case, it is preferable that a projecting tapered section isformed on the leading end of one of the first shaft section or thesecond shaft section, and a corresponding recessed tapered portion isformed in the residual leading end of the other. According to theforegoing preferred structure, by moving the second substrate supportsection downwards, the projecting tapered portion formed on the leadingend of either of the first shaft section or the second shaft section andthe recessed tapered portion formed in the leading end of the other areengaged to each other. Thus, the central axis of the first informationlayer and that of the second information layer can be made to coincidewith each other.

According to another aspect of the present invention, there is providedan apparatus for optically recording and reproducing information whichirradiates, with light, an optical information recording medium havingtwo different information layers provided with one or more selected fromthe group consisting of information pits corresponding to informationsignals, guide grooves for tracking and sample pits so as to be capableof reproducing information signals from the information layers,recording information signals on the information layers and reproducingthe recorded information signals. The apparatus for optically recordingand reproducing information comprises: optical means for causing anobjective lens to converge a light beam emitted from a light source ontothe recording medium; focus control means for performing control to makethe focal point of the light beam coincide with either of theinformation layers; tracking control means for controlling the positionof the light beam to enable the light beam to follow the informationpits, guide grooves or the sample pits; layer identification means fordemodulating a signal for identifying the information layers inaccordance with light reflected by or transmitted through theinformation pits; layer selection means for selecting an informationlayer from which the information signal is reproduced or on which thesame is recorded; and switch means for switching a method of tracking,which is performed by the tracking control means, to correspond to aresult of selection performed by the layer selection means. According tothe foregoing preferred structure of the an apparatus for opticallyrecording and reproducing information, information can be recorded onand reproduced from the recording medium having a multilayer structure.

It is preferable that the apparatus for optically recording andreproducing information according to the present invention furthercomprises layer comparison means for subjecting, to a comparison aresult of selection performed by the layer selection means and a resultof identification performed by the layer identification means, and afocus jumping circuit for generating a pulse voltage to move the focalpoint of the light beam between the information layers to correspond toan output from the comparison means. According to the foregoingpreferred structure, if an information layer, which is not the subject,is focused, the focusing position can be moved to the subjectinformation layer. In the foregoing case, it is preferable further toprovide tracking polarity inverting means for switching the polarity ofthe tracking control means in synchronization with the operation of thefocus jumping circuit. According to the foregoing preferred structure,even in a case of a recording medium having the first information layerand the second information layer having information pits formed in theopposite directions between the two layers when viewed from a positionupon which the light beam is made incident, the light beam caninstantaneously be moved onto the information pit of the subjectinformation layer.

It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the focus control means includes a first focuscontrol means for performing control to make the focal point of thelight beam to coincide with a position near the information layer,second focus control means permitted to be operated in a range smallerthan that of the first focus control means, and focus switch means forperforming switching to the second focus control means after theoperation of the first focus control means has been completed. Accordingto the foregoing preferred structure, servo operation of eachinformation layer can be performed stably while maintaining the focuspulling performance similar to that obtainable from the conventionalstructure.

It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the focus control means has at least two types ofoperation conditions to be adaptable to each of the information layers,the focus control means being arranged to select one of the operationconditions in accordance with a result of selection performed by thelayer selection means. According to the foregoing preferred structure,focus deviation can be corrected so that information can be recorded orreproduced satisfactorily.

It is preferable that the apparatus for optically recording andreproducing information according to the present invention has astructure such that the tracking control means has at least two types ofoperation conditions to be adaptable to each of the information layers,the tracking control means being arranged to select one of the operationconditions in accordance with a result of selection performed by thelayer selection means. According to the foregoing preferred structure,tracking deviation can be corrected so that information can be recordedor reproduced satisfactorily.

It is preferable that the apparatus for optically recording andreproducing information according to the present invention furthercomprises a photodetector for performing a focus control including afirst divisional light receiving surface for receiving a portion oflight reflected by the recording medium and a second divisional lightreceiving surface for receiving the reflected light on the same plane onwhich the first divisional light receiving surface is positioned, thesecond divisional light receiving surface receiving the reflected lighton the outside of the first divisional light receiving surface.According to the foregoing preferred structure, when information isreproduced from a recording medium having a multilayer structure, therange in which the focus is pulled can be enlarged by switching thefocus detection region between the moment when the focus is pulled andthe moment when the servo operation is performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing the structure of an opticalinformation recording medium having two information layers according tothe present invention;

FIG. 2(a) and (b) are perspective views showing the structure of anoptical information recording medium having two reproduction onlyinformation layers according to the present invention;

FIG. 3 is a perspective view showing the structure of an opticalinformation recording medium having a reproduction only informationlayer and a recording and reproducing information layer according to thepresent invention;

FIG. 4 is a perspective view showing the structure of an opticalinformation recording medium having two recording and reproducinginformation layers according to the present invention;

FIG. 5 is a cross sectional view showing the structure of an opticalinformation recording medium having four information layers according tothe present invention;

FIG. 6 is a schematic cross sectional view showing an apparatus formanufacturing the optical information recording medium according to thepresent invention;

FIG. 7 is a first partial cross sectional view showing the apparatus formanufacturing the optical information recording medium according to thepresent invention;

FIG. 8 is a second partial cross sectional view showing the apparatusfor manufacturing the optical information recording medium according tothe present invention;

FIG. 9(a)-(e) show the sequence for manufacturing the opticalinformation recording medium having two information layers according tothe present invention;

FIG. 10(a)-(j) show the sequence for manufacturing the opticalinformation recording medium having four information layers according tothe present invention;

FIG. 11 is a block diagram showing the structure of an apparatus foroptically recording and reproducing information according to the presentinvention;

FIG. 12 is a schematic view showing the structure of an optical pickupof the apparatus for optically recording and reproducing informationaccording to the present invention;

FIG. 13 is a schematic view showing the structure of a focus controlsection of the apparatus for optically recording and reproducinginformation according to the present invention;

FIG. 14(a)-(d) are graphs showing the waveforms of focus error signalsobtainable from the two information layers of the optical informationrecording medium according to the present invention;

FIG. 15 is a schematic view showing a photodetector of the apparatus foroptically recording and reproducing information;

FIG. 16 is a schematic view showing the structure of a focus controlsection of the apparatus for optically recording and reproducinginformation according to the present invention;

FIG. 17 is a schematic view showing the structure of a tracking controlsection of the apparatus for optically recording and reproducinginformation according to the present invention;

FIG. 18 is a cross sectional view showing another example of the opticalinformation recording medium having a reproduction only informationlayer and a recording and reproducing information layer according to thepresent invention;

FIG. 19 is a cross sectional view showing still another example of theoptical information recording medium having a reproduction onlyinformation layer and a recording and reproducing information layeraccording to the present invention;

FIG. 20 is cross sectional view showing another example of the opticalinformation recording medium having two recording and reproducinginformation layers according to the present invention; and

FIG. 21(a)-(f) show the process for manufacturing a conventional opticalinformation recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An optical information recording medium and an optical informationrecording and reproducing apparatus according to the present inventionwill now be described with reference to the drawings.

FIG. 1 is a cross sectional view showing an embodiment of the opticalinformation recording medium according to the present invention. Asshown in FIG. 1, guide grooves for tracking, sample pits or informationpits corresponding to information signals are formed on either surfaceof a first substrate 1 having thickness d1. On the foregoing surface ofthe first substrate 1, there is formed a first information layer 2formed by a thin film which reflects a portion of a light beam 7 madeincident on the first substrate 1 and permits penetration of a portionof the light beam 7, the thin film 2 having thickness d2. On a surfaceof a second substrate 3 having thickness d3, there are formed guidegrooves or information pits corresponding to information signals. On thesurface of the second substrate 3, there is formed a second informationlayer 4 formed by a thin film having a reflectance higher than that ofthe first information layer 2 and having thickness d4. Between the firstinformation layer 2 and the second information layer 4, there is formeda transparent separation layer 5 for positioning the first informationlayer 2 and the second information layer 4 apart from each other for apredetermined distance d5. Thus, an optical information recording mediumhaving a double-layer structure is constituted.

It is preferable that the first substrate 1 and the second substrate 3are formed vertically symmetrically with respect to the separation layer5 as much as possible. That is, it is preferable that the materials andthe thicknesses (d1 and d3) are made to be substantially the same andthe two substrates are different from each other in only the pattern ofthe information pits on the surfaces thereof and the structures of thefirst and second information layers 2 and 4.

If the optical information recording medium is constituted as describedabove, a vertically symmetric structure with respect to the separationlayer 5 can be formed. Therefore, even if a stress or the like isgenerated in the substrate due to change in the temperature or the likeduring the manufacturing process, any generated distortion can becompensated. Even if unusual deformation factors act on the twosubstrates due to change in the environment temperature or humidity,deformation and warp can be inhibited. As a result, a structure durableagainst change in the environment can be obtained. Therefore, even ifthe first and second substrates 1 and 3 are made of a resin, deformationof the manufactured recording medium causing an error to take place inreproducing signals can be prevented.

Information signals can be reproduced from the optical informationrecording medium according to this embodiment in such a manner that thetwo information layers (the first and second information layers 2 and 4)are irradiated with the light beam 7 from a position on the outside ofthe first substrate 1, and then change in the quantities of reflectedlight beams is detected so that the information signals recorded on thefirst and second information layers 2 and 4 are reproduced. To enablethe information signals to be reproduced, the light beam 7 forirradiation must be converged efficiently onto each of the first andsecond information layers 2 and 4.

Therefore, the first information layer 2 must have a predeterminedreflectance to enable the information signals formed on the firstinformation layer 2 to be reproduced as the change in the reflectedlight. Moreover, to allow the light beam 7 for irradiation and having apredetermined intensity to reach the second information layer 4, thefirst information layer 2 must have a predetermined transmissivity.Although the second information layer 4 is not required to a specifictransmissivity, the second information layer 4 must have the highestpossible reflectance to enlarge the change in the quantity of reflectedlight to provide the information signal. That is, when the informationsignals of the second information layer 4 are reproduced, the light beam7 must, penetrate the first information layer 2 two times. Therefore,this embodiment has the structure such that the reflectance of thesecond information layer 4 is set to be higher than that of the firstinformation layer 2.

As a material of the first and second substrates 1 and 3, it ispreferable that a material be employed which does not considerablyabsorb light in a wavelength region of the light beam 7 for theirradiation and which has great strength. Accordingly, the material ofthe first and second substrates 1 and 3 is a resin, such as apolycarbonate resin, polymethylmethacrylate (PMMA) resin or the like, orglass.

In the case where the resin is used as the material of the first andsecond substrates 1 and 3, the substrate can be formed by heating theresin to the molten state and filling it into a mold having informationpits or a guiding groove on one side thereof. In the case where glass isused as the material of the first and second substrates 1 and 3, amethod is employed in which information in the form of information pitsis formed on the surface of a flat glass plate by, for example, etching.As an alternative to this, a method (a Photo-Polymerization method) isemployed in which an ultraviolet curing resin is applied to the surfaceof the flat glass plate, followed by pressing a die having a surfacecomprising information pits to form information in the form ofinformation pits. However, the methods of forming the first and secondsubstrates 1 and 3 are not limited to the foregoing methods. Any methodmay be employed as long as the employed method is able to give apredetermined optical characteristic to the substrates. Since theforegoing methods are known methods employed in a process formanufacturing a usual optical disk, such as a compact disk, furtherdescription thereof is omitted.

The patterns of convex and concave portions to be formed on the surfacesof the first and second substrates 1 and 3 are different from each otherdepending upon whether the function of the information layers is anexclusively reproducing type or a recording and reproducing type. In thecase where the information layers are arranged to be exclusivelyreproducing, the convex and concave portions are in the form ofinformation pit columns constituted by a pattern formed on the surfacesof the substrates and modulated in response to the information signals.In the case where the information layers are arranged to be recordingand reproducing, the convex and concave portions are in the form ofguide grooves consisting of continuous convex and concave portions forperforming tracking control of the light beam or wobble pits adaptableto a tracking method, called a "sample servo method".

It is preferable that the first and second substrates 1 and 3 havesubstantially the same size, and are formed by using the same materialin the same process. In particular, if a resin material is employed asthe material of the substrates and if the substrates are formed by theinjection molding method, the substrates can become deformed, such aswarped, after being allowed to stand for a long time depending upon themolding conditions or the like. Moreover, change in the environmenttemperature or humidity causes the substrates to be considerablydeformed. In the case where the substrates are formed by thePhoto-Polymerization method, similar deformation takes place, though thedegree of the deformation is not critical as is experienced with theinjection molding method. While considering the foregoingcharacteristics, this embodiment has the structure such that the firstand second substrates 1 and 3 are formed in a similar process and thetwo substrates are adhesive-bonded to each other with the separationlayer 5. As a result of the employment of the vertically symmetricstructure with respect to the separation layer 5, stress and distortionof the substrates can be inhibited. Thus, an optical informationrecording medium having a satisfactory durability against change in theenvironment can be obtained.

The thicknesses of the two substrates, with which substantially the samemechanical strengths can be attained, depend upon the temperature of theenvironment in which the recording medium exists or the material of thesubstrates. To obtain an allowance of the difference in the thicknessesof the two substrates, an optical information recording medium havingthe following structure was manufactured: a polycarbonate resin having athickness of 0.6 mm was employed to form the first substrate 1. Then, anAu film having a thickness of 10 nm was formed on the first substrate 1so that a first information layer 2 was formed. The thickness of thesecond substrate 3 made of the polycarbonate resin was changed from 0.3mm to 1.2 mm, and then an Au film having a thickness of 100 nm wasformed so that a second information layer 4 was formed. Moreover, aseparation layer 5 was formed by an acrylic-type ultraviolet curingresin layer having an average thickness of 40 μm. The separation layer 5was used to adhesive-bond the first information layer 2 and the secondinformation layer 4 to each other. The quantity of deformation of thethus-manufactured optical information recording medium was measured,thus resulting in the following values being obtained. That is, if thethickness of the second substrate 3 is 0.6 mm±30% or smaller, allowingto stand in a room temperature environment, the temperature of which was30° C. and the relative humidity (RH) of which was 80% for 1000 hours,resulted in a quantity of warp of the recording medium of 0.4 mm orsmaller. In this case, a stable servo operation was possible. Even ifthe optical information recording medium was allowed to stand in a moresevere environment, the temperature of which was 80° C. and the relativehumidity of which was 80% for 1000 hours, warp of the recording mediumcould be inhibited in a case where the thickness of the second substrate3 is 0.6 mm±20% or smaller.

The information layer is classified into two types exemplified by areproduction-only type and recording and reproducing type. The recordingmedium according to this embodiment has two information layers.Therefore, the structure of the recording medium may be, in thesequential order as the first information layer and the secondinformation layer, any of the following four types, that is, (A){reproduction only}--{reproduction only}, (B) {reproductiononly}--{recording and reproducing}, (C) {reproducing andreproducing}--{reproduction only}, and (D) {recording andreproducing}--{recording and reproducing}.

The reproduction only information layer is formed by a thin film formedon the surface of the substrate having the foregoing information pitsformed thereon, the thin film having a predetermined reflectance withrespect to the light beam. In the foregoing case, a material may beselected from a group consisting of metal, such as Au, Al, Cu or theiralloys; an oxide, such as SiO₂, SiO, TiO₂, MgO or GeO₂ ; a nitride, suchas Si₃ N₄ or BN; a dielectric material of a sulfide, such as ZnS or PbS;their mixtures; and a multilayer structure of the foregoing oxide,nitride and the sulfide. By using the foregoing material, informationlayers having a predetermined reflectance with respect to the light beamhaving a specific wavelength can be obtained.

In the case where the first information layer 2 is a reproduction onlyinformation layer, the first information layer 2 must have apredetermined transmissivity with which the first information layer 2reflects the light beam 7 made incident upon from a position on theoutside of the first substrate 1 and allows the light beam having apredetermined intensity to reach the second information layer 4. In thecase where the same material is used to form the first information layer2 and the second information layer 4, the foregoing object can beobtained by making the thickness of the first information layer 2 to bethinner than that of the second information layer 4. In the case where ametal is employed to form the information layer, the metal is formedinto a thin film having a thickness of 5 nm to 40 nm. To maintain bothreflectance and the transmissivity of the first information layer 2 athigh levels, it is preferable that absorption of light by theinformation layer is the lowest possible level. In the foregoing case,the first information layer 2 may be a dielectric material or an organicmaterial capable of realizing a high refractive index and a lowcoefficient of absorption. Moreover, a layer formed by stacking thedielectric material and the organic material enables an informationlayer that does not considerably absorb light to be obtained.

In the case where the second information layer 4 is the reproductiononly information layer, the transmissivity is not required to beconsidered. It is preferable that the reflectance be the highestpossible reflectance. In the case where the metal is employed to formthe second information layer 4, a metal formed into a thin film having athickness of 40 nm to 200 nm is employed.

The recording and reproducing type information layer comprises a thinfilm formed on the substrate having the guide grooves or the sample pitsformed thereon, the thin film having the optical characteristic which ischanged when it absorbs the light beam for the irradiation and the stateof the change being identified with the light beam. As the recordinglayer for use as the information layer, a material to be employed may beselected from the group which consists of a phase-change material, thereflectance of which is changed due to light for the irradiation becausethe state of the thin film is changed; a magneto-optic material in theform of the thin film, the direction of the magnetization of which ischanged and the change of which can be detected as a Kerr effect; anorganic material, such as a coloring matter, having the spectralreflection factor which is changed; and a photochromic material.

The phase-change material, the phase of which is changed betweenamorphous and crystal, may be selected from a group consisting of achalcogen material, such as SbTe, InTe, GeTeSn, GeSbTe, SbSe, TeSeSb,SnTeSe, InSe, TeGeSnO, TeGeSnAu or TeGeSnSb type material; and an oxidematerial, such as Te--TeO₂, Te--TeO₂ --Au or Te--TeO₂ --Pd typematerial.

The phase-change material, the phase of which is changed between crystaland crystal, may be a metal compound, such as an AgZn compound or anInSb compound.

As the magneto-optic material, MnBi, TbFe or TbFeCo type material may beemployed.

As the organic coloring matter, a leuco dye, such as triphenylmethane orthe like may be employed. The photochromic material may be spiropyran,fulgide or azo type material.

Note that the recording-enabled information layer is, in view of itsfunction, classified into a write-once type information layer to whichinformation can be recorded only one time and a rewriting typeinformation layer on which recorded information can be rewritten. In thecase of the write-once type information layer, only one layer made ofthe phase-change material layer or the organic coloring matter layer isrequired to be, as the information layer, formed on the substrate.Another method may be employed in which a double-layer structureconsisting of a light absorbing thin film layer and a metal layer isemployed to prepare alloys by irradiation with light.

Although the information layer may be constituted by only a recordinglayer, it is preferable that a plural-layer-structure including at leasttwo layers be employed in order to cause the material forming theinformation layer to be reversibly changed and the optical change in therecorded signal to be enhanced. The double-layer structure may be astructure including a dielectric material layer/a recording layer, astructure including a recording layer/a reflecting layer or a structureincluding a reflecting layer/a recording layer (in the foregoingsequential order when viewed from a position upon which the light beam 7is made incident). A triple-layer structure may be, when viewed from thesubstrate, a structure including a dielectric material layer/a recordinglayer/a dielectric material layer or a structure including a dielectricmaterial layer/a recording layer/a reflecting layer. A quadruple layerstructure may be a structure including, when viewed from a position uponwhich the light beam 7 is made incident, a dielectric material layer/arecording layer/a dielectric material layer/a reflecting layer. Aquintuple structure may including a first reflecting layer/a dielectricmaterial layer/a recording layer/a dielectric material layer/a secondreflecting layer when viewed from the substrate. By forming therecording layer and the dielectric material layer to be in contact witheach other, deterioration in the thin film when recording is performedrepeatedly can be prevented. Moreover, optical change in recordedinformation can be set to a great degree.

The dielectric material layer may be made of a material selected from agroup consisting of an oxide, such as SiO₂, SiO, TiO₂, MgO or GeO₂ ; anitride, such as Si₃ N₄ or BN; a sulfide, such as ZnS or PbS; and theirmixtures.

The reflecting layer may be made of any material exemplified when thereproduction only information layer has been described.

In order to maintain a sufficiently large quantity of light on thesecond information layer 4, it is preferable that the separation layer 5be made of a material which does not considerably absorb light havingthe wavelength region of the light beam 7, in particular, light whichhas passed through the first information layer 2. Therefore, theseparation layer 5 may be made of a transparent adhesive agent, glasssimilar to that of the substrate or a resin material. In the case wherethe first and second substrates 1 and 3 are made of the resin material,it is preferable that a similar-type resin material be employed tomaintain mechanical reliability after adhesive-bonding. To shorten thetime required to complete the adhesive-bonding process, it is preferablethat a ultraviolet curing resin be employed.

The distance d5 of the separation layer 5 must be at least longer thanthe focal depth determined by the numerical aperture (NA) of anobjective lens 6 and the wavelength (λ) of the light beam 7 in order toinhibit influence of the crosstalk from another information layer wheneither information layer is being reproduced. If the intensity of thelight convergent point is 80% or greater with respect to the centralintensity (100%) when a light beam is coverged in a stigmatic case, thefocal depth Δz allows approximation with the following Equation (1):

    Δz=λ/{2(NA)).sup.2 }                          (1)

In an exemplary case where λ=780 nm and the NA=0.55, then Δz=1.3 μm.Therefore, a region within ±1.3 μm is included in the focal depth. Inthe case where the foregoing optical system is employed, it ispreferable that the thickness d5 of the separation layer 5 be set to avalue larger than 2.6 μm.

The influences of recording marks included in the light beam passingthrough the first information layer 2 when the light beam 7 is focusedto the second information layer 4, act as crosstalk taking place whenthe second information layer 4 is reproduced. Therefore, in order toreproduce the signals stably, it is preferable that the thickness d5 ofthe separation layer 5 be at least longer than the focal depth, morepreferably five times the focal depth. A usual optical disk of areproduction-only type has information pits formed on the opticalrecording medium at a pitch shorter than the focal depth. If thethickness of the separation layer is made to be five time the focaldepth, the number of the information pits on the first information layer2, which is irradiated with the light beam 7, is 25 or more, which issufficiently smaller than a usual allowance of -26 dB to prevent thecrosstalk.

To maintain the high recording density of information to be formed onthe first and second information layers 2 and 4, the first and secondinformation layers 2 and 4 must be formed in a range into which theobjective lens 6 is able to converge light beams. That is, the value ofd1+d5, which is the result of addition of the thickness d5 of theseparation layer 5 to the thickness d1 of the first substrate 1, must bewithin a tolerance for the thickness of the base which is permitted bythe optical system (the objective lens 6).

Therefore, it is preferable that the thickness d5 of the separationlayer 5 be larger than the focal depth of the optical system forconverging the light beams 7 and smaller than the tolerance for the basepermitted by the foregoing optical system. If the foregoing conditionsare satisfied, information, which is not considerably affected by thecrosstalk from an information layer other than the subject informationlayer, can be reproduced when the aberration of the light beam 7 issmall. Note that the thickness d5 of the separation layer 5 must be setto an optimum value in consideration of the yield when the opticalinformation recording mediums are mass-produced as recording mediums aswell as the optical aberration. By forming the first and secondsubstrates 1 and 3, the first and second information layers 2 and 4 andthe separation layer 5 by using the foregoing materials, a recordingmedium can be obtained which allows information signals to be reproducedfrom the first and second information layers 2 and 4 when theinformation layers 2 and 4 are irradiated with light from a position onthe outside of the first substrate 1.

A structure of a recording medium capable of stably and easilyreproducing signals from the two information layers will now bedescribed. To reproduce information signals recorded on the twoinformation layers stably and easily, it is preferable that the levelsof signals obtainable from the two information layers be similar to eachother also in view of simplifying the structure of the reproducingapparatus. The description will hereinafter be directed to a structurein which the quantities of reflected light obtainable from the flatportions of the two information layers are similar to each other whenviewed from a position upon which the light beam 7 is made incident. Inorder to facilitate the approximation, the description will be directedto the structure in which both of the first and second informationlayers 2 and 4 are reproduction only information layers. Note that anassumption is made that the influence of diffraction of transmittedlight occurring due to the information pits of the first informationlayer 2 can be ignored.

An assumption is made that the reflectance of the first informationlayer 2 is R1, the absorption ratio of the same is A1 and thereflectance of the second information layer 4 is R2. In the foregoingcase, another assumption is made that the quantities of reflected lightfrom the flat portions of the two information layers are the same whenthe amplitudes of signals from the two information layers are the same.The foregoing fact is equivalent to a fact that the quantity T of alight beam 7 that has been incident and has penetrated the firstinformation layer 2 and been reflected by the second information layer 4and then penetrated the first information layer 2 again, and thereflectance R1 of the first information layer 2 are the same. In theforegoing case, the relationships expressed by the following equationshold:

    R1≈T                                               (2)

    R1≈(1-A1-R1).sup.2 ×R2                       (3)

    R1≈1-A1+(2·R2).sup.-1 -{ 1-A1+(2·R2).sup.-1 !.sup.2 -(1-A1).sup.2 }.sup.0.5                           (4)

R1 is made to be a maximum value when R2=1 and A1=0. In this case, R1 is0.382. If R2=0.9 and A1=0.1 in a practical view point, then R1=0.311.

The foregoing phenomenon means that, if the diffraction of lighttransmitted through the first information layer 2 is ignored and thedegrees of diffraction of light reflected by the first and secondinformation layers 2 and 4 are the same, the forming of a secondinformation layer 4 having a reflectance R2 of 90% and a firstinformation layer 2 having a reflectance R1 of 31% and an absorptioncoefficient A1 of 10% results in the amplitudes of signals to be madethe same when information signals are reproduced from the informationpits of the first and second information layers 2 and 4.

Referring to Equation (2), a practical structure of the two informationlayers will now be described. An assumption is made here that the rangein which the reproduction amplitudes from the two information layers canbe considered to be equivalent is ±20%. The foregoing fact means thatthe difference between the right side and the left side of Equation (2)is ±20% or smaller.

Thus, substitution of Equation (3) with the foregoing relationshipresults in the relationship expressed by the following Equation (5)being holding in a case where the reproduction amplitude from the firstinformation layer 2 is smaller by 20% than that from the secondinformation layer 4. On the other hand, in a case where the reproductionamplitude from the first information layer 2 is larger by 20% than thatfrom the second information layer 4, the following Equation (6) hold:

    R1=1.2×(1-A1-R1).sup.2 ×R.sup.2                (5)

    R1=0.8×(1-A1-R1).sup.2 ×R.sup.2                (6)

While considering the practical characteristic of each layer in the casewhere the information layers are made of metal or the dielectricmaterial, it is preferable that the reflectance R2 of the secondinformation layer 4 be within a range from 70% to 90%. Moreover, it ispreferable that the refractive index A1 of the first information layer 2be within a range not greater than 20%. Substituting portions ofEquations (5) and (6) with the foregoing relationships, the range of thereflectance R1 of the first information layer 2 is 21% to 42%. Tomaintain the great reproduction amplitudes from both of the first andsecond information layers 2 and 4, it is preferable that the absorptioncoefficient A1 of the first information layer 2 be small. If the valueof the absorption coefficient A1 is 10% or smaller, the range of thereflectance R1 of the first information layer 2 is 25% to 40%.

As described above, in the case where both of the first and secondinformation layers 2 and 4 are reproduction only information layers, itis preferable that the reflectance R1 of the first information layer 2be within a range of 25% to 40% to make the reproduction amplitudes fromthe first and second information layers 2 and 4 to be similar to eachother and to maintain the large reproduction amplitude.

In the case where the second information layer 4 is a recording andreproducing information layer, the reflectance of the information layeris smaller than that of the reproduction only information layer. In acase where the reflectance of the second information layer 4 is 30%, useof the first information layer 2 having a reflectance of 19.5% and acoefficient of absorption of 0% makes the quantities of light reflectedtoward the objective lens 6 to be the same when both of the informationlayers are irradiated with light. Although the structure of therecording and reproducing apparatus will be described later, theamplitude of the reproduced signal and the quantity of reflected lighthave predetermined allowances in consideration of the difference betweenrecording mediums and contamination of the surfaces of the recordingmediums. Therefore, if the accuracy of the reproducing circuit and thestability of the apparatus are considered, it is preferable that thedifference between the amplitudes of the two reproducing signals besmaller than 5 times.

The present invention is characterized in that the substrates havingconvex and concave portions formed previously are adhesive-bonded toeach other so that a recording medium having two information layers isobtained. Therefore, the information pits pattern to be formed on thesurface of the first substrate 1 must have signal shapes that can bereproduced through the base of the first substrate 1. Moreover, theinformation pits pattern to be formed on the surface of the secondsubstrate 3 must have signal shapes that can be reproduced whenirradiated with light through the surfaces of the information pits.Accordingly, the information pits pattern of the first and secondsubstrates 1 and 3 are formed to run in the same direction when viewedfrom a position upon which the light beam 7 is made incident. In thecase where the information layers are the reproduction only informationlayers, the direction of the information pits pattern is made to be inthe direction in which the information pits are formed to correspond tothe information signals. In the case where the information layers arethe information recording and reproducing layers, the foregoingstructure is applied to address information for administering the guidegrooves.

The structure of the recording medium will now be described in detail inwhich the first and second information layers 2 and 4 are a reproductiononly information layer and a reproduction only information layercategorized as (A).

In view of reducing the manufacturing cost of the substrates, it ispreferable that the first and second substrates 1 and 3 be manufacturedby the same process as much as possible. The information pits are formedon the surfaces of the substrates by a method including of a masteringstep for manufacturing a master and an injection molding step in which aresin material is injected into the master placed in a mold to form asubstrate having information pits. Since the mastering method is a knownmethod which is usually employed when compact disks or CD-ROMs aremanufactured, the detailed description of this method is omitted here.Briefly, a photoresist is applied to a flat glass plate, following byirradiating the surface of the photoresist with Ar laser beams modulatedin response to the information signals, followed by removing thephotoresist, and followed by plating the surface from which thephotoresist has been removed so that a master is manufactured.

If the same mastering process is employed, the shapes of the informationpits to be formed on the substrates, that is, the relationships of theinformation pits whether they are formed in the form of convex andconcave portions with respect to a plane, are made to be the same. FIG.2(a) shows an example of an optical information recording mediummanufactured by adhesive-bonding the substrates formed by the sameprocess. As shown in FIG. 2(a), information pits 11 of the firstsubstrate 1 have convex shapes when viewed from the position upon whichthe light beam 7 is made incident. Information pits 12 of the secondsubstrate 3 have concave shapes when viewed from the position upon whichthe light beam 7 is made incident. In the case where the characteristicsof the photoresists are different from each other, information pits ofthe first substrate 1 have concave shapes when viewed from the positionupon which the light beam 7 is made incident. Information pits of thesecond substrate 3 have convex shapes when viewed from the position uponwhich the light beam 7 is made incident. In any case, the directions ofthe information pits formed in the two information layers 2 and 4 areopposite to each other when viewed from the position upon which thelight beam 7 is made incident.

As a result of the foregoing structure, the same material and moldingprocess can be employed as well as the mastering process. Therefore, twotypes of manufacturing apparatuses having the same function are requiredto be prepared when the optical information recording medium ismanufactured or one manufacturing apparatus is required to be usedcommonly. Therefore, the cost of the manufacturing facilities can bereduced.

Since the recording medium having the foregoing structure compris thepits formed in the opposite directions between the two informationlayers 2 and 4 when viewed from a position upon which the light beam 7is made incident, the tracking polarity must be switched between theinformation layers 2 and 4 when a tracking method, such as a push-pullmethod, is used to record or reproduce information. To prevent this, thedirection of the pits of the second information layer 4 is required tobe reversed to have convex shapes when viewed from the position uponwhich the light beam 7 is made incident. By manufacturing the secondinformation layer 4 with a master having the photoresist with thereversed characteristic in the mastering process or by using a secondmaster obtainable by again transferring a master obtained by theconventional method, pits formed in the opposite directions can beobtained between the two information layers 2 and 4.

The sizes of the information pits of the first and second substrates 1and 3 will now be described. The size of the pits to be formed on thesecond substrate 3 is classified into two types depending upon whetherthe distance from the surface of the first substrate 1 upon which thelight beam 7 is made incident to the surfaces of the first and secondinformation layers 2 and 4 is within tolerance ΔWd of the thickness ofthe base permitted by the optical system for converging the light beam7. Note that the tolerance ΔWd of the thickness of the base isdetermined by the spherical aberration of the light beam 7, thetolerance ΔWd of the thickness of the base being generally in inverseproportion to the fourth power of the numerical aperture (NA) of theobjective lens 6 (see FIG. 1). For example, in an optical system havinga wavelength λ of 780 nm and a numerical aperture NA of 0.5, thetolerance ΔWd of the thickness of the base is about 50 μm. Note that thetolerance ΔWd of the thickness of the base depends upon the density ofthe pits, that is, the intervals between the pits. If the intervalsbetween the pits are long, signals can be reproduced even if a sphericalaberration takes place. Thus, the tolerance range is enlarged.

A structure is shown in FIG. 2(a) which is employed in the case whereboth of the distances from the surface of the first substrate 1 uponwhich the light beam 7 is made incident to the surfaces of the first andsecond information layers 2 and 4 are within the tolerance ΔWd of thethickness of the base determined by the light converging optical systemand the pit density. A main issue in the foregoing case lies in that theinformation pits 11 on the first information layer 2 and secondinformation layer 4 are different from each other. The reason for thisis that the first information layer 2 has a main reflecting surfacewhich is in contact with the first substrate 1, while the secondinformation layer 4 has a main reflecting surface which is the interfacebetween the second information layer 4 and the separation layer 5.

In a case where the width of each of the information pits 11 on thefirst substrate 1 is W11 and the width of each of the information pits12 on the second substrate 3 is W12, the width of the main reflectingsurface is such that the width is the pit width W11 for the firstinformation layer 2 and the same is the pit width W13 for the secondinformation layer 4, the pit width W13 being the width of the pit of theinterface between the second information layer 4 and the separationlayer 5. In a case where a known sputtering method is employed to formthe information layers on the substrates, the thin film will be formedto reach the diagonal surfaces of the information pits as well as in adirection perpendicular to the surface of the substrate though thedegree depends upon the manufacturing method. Therefore, the pit widthW13 on the interface with respect to the separation layer 5 is smallerthan pit width W12 of the information pits 12 on the second substrate 3.To make the degrees of diffraction of light reflected by the informationpits to be the same, the pit width W12 of the information pits 12 on thesecond substrate 3 must be larger than the pit width W11 of theinformation pits 11 on the first substrate 1.

The pit width W12 of the information pits 12 on the second substrate 3is corrected in accordance with a result of a process of actuallyforming the second information layer 4. The inventors have performed anexperiment in which Au was used to form the second information layer 4.On information pits each having a pit width W12 of 0.50 μm and a depthof 90 nm on the second substrate 3, there was formed an Au layer (thesecond information layer 4) having a reflectance of 90% or higher and athickness of 150 nm so that the shape of the second information layer 4,which corresponds to the interface with respect to the separation layer5, was measured. As a result, the pit width W13 was 0.3 μm and the depthwas 90 nm. In a case where the second information layer 4 is formedunder the foregoing conditions, the shapes of the information pit to beformed on the surface of the second substrate 3 is determined such thatthe pit width W12 is 0.70 μm in consideration of the change in the shapeof the pit due to existence of the second information layer 4. In theforegoing case, information pits each having a pit width W11 of 0.50 μmand a depth of 90 nm are formed on the first substrate 1. As describedabove, the information pits 12 on the second substrate 3 are formed tohave a size larger than that of the information pits on the firstsubstrate 1 in consideration of a reduction in the substantial width ofthe information pit due to the thickness of the second information layer4. Note that track pitches Tp1 and the pit densities of the informationpits to be formed on the surfaces of the first and second substrates 1and 3 are the same.

In the mastering process for forming the second substrate 3, the size ofthe information pit is enlarged as compared with that on the firstsubstrate 1 by setting the power of a light source for exposing thephotoresist to light to be a slightly larger value. The other processesfor forming the first and second substrates 1 and 3 are the same.

Although the case has been described in which the pit widths were madeto be different from each other between the first and second substrates1 and 3, the conditions for forming the information layers sometimesresult in a consideration being made such that the angle of inclinationof the diagonal surface of the pits on the substrates is different fromthe angle of inclination of the diagonal surfaces of the formedinformation layers. In the foregoing case, the depth of the pit on thefirst substrate 1 and that on the second substrate 3 are made to bedifferent from each other, or both of the pit width and the pit depthare made to be different between the same. As a result of the foregoingstructure, the degrees of diffraction of reflected light with respect tothe incidental light beam 7 taking place between the first and secondinformation layers 2 and 4 can be made to approach each other. Thus,stable signal reproduction can be performed.

FIG. 2(b) shows a structure to be employed in a case where either of thetwo information layers is out of the tolerance of the thickness of thebase permitted by the light converging optical system. A main issue inthe foregoing case lies in that convergence of the light beam 7 to aninformation layer which is out of the tolerance ΔWd of the thickness ofthe base results in a spherical aberration being generated and thussufficient convergence of light spots being inhibited. The phenomenon inwhich either of the two information layers is out of the tolerance ΔWdof the thickness of the base of the light convergence optical systemtakes place due to reduction in the tolerance ΔWd of the thickness ofthe base occurring when the wavelength of the light beam 7 has beenshortened and the numerical aperture (NA) of the objective lens has beenenlarged for the purpose of raising the density of information on theinformation layer. The foregoing phenomenon occurs in a case where athin separation layer cannot easily be obtained when the separationlayer 5 is formed or in a case where the accuracy of the thickness ofthe separation layer 5 is unsatisfactory.

In the foregoing case, the density of pits on the surface of thesubstrate having the information layer which is out of the tolerance ΔWdof the thickness of the base is required to be lower than the density ofthe pits on the substrate having the information layer which is withinthe tolerance ΔWd of the thickness of the base. FIG. 2(b) shows anexample of a structure to be employed in the case where the firstinformation layer 2 is within the tolerance ΔWd of the thickness of thebase and the second information layer 4 is out of the tolerance ΔWd ofthe thickness of the base. Each of information pits on the firstsubstrate 1 has a predetermined pit width W11, a track pitch of Tp1 anda pit density of Pd1. Each of information pits 14 on the secondsubstrate 3 has a predetermined pit width W14, a track pitch of Tp3 anda pit density of Pd3. While considering deterioration in the diaphragmdue to the spherical aberration of the light beam 7 on the secondinformation layer 4, the pit width W14 and the track pitch Tp3 of theinformation pits 14 on the second substrate 3 are made to be larger thanthose of the first substrate 1.

As a result of the foregoing structure, even if the second informationlayer 4 is out of the tolerance ΔWd of the thickness of the base of theoptical system for converging the light beam 7, a change in the quantityof reflected light similar to that obtainable from the first informationlayer 2 can be obtained from the pit portions. Thus, stable signalreproduction can be performed.

Then, a pattern of the information pits or that of the guide grooves onthe surfaces of the substrates in the direction of the track will now bedescribed. Although the information pits to be formed on the first andsecond substrates 1 and 3 may be formed into concentric circles, it ispreferable that a spiral form be employed similar to that of theconventional optical disk in view of capability that the accuracy of thetrack pitch and the like can be improved as compared with the structurehaving the concentric circles when the mastering process is performed.The structure having the spiral projection and pit column is classifiedinto two types to meet the purpose.

A first structure is arranged such that the projection and pit columnson the first and second substrates 1 and 3 run in the same directionwhen viewed from the light incidental portion. In the foregoing case,the light beam is moved in one direction from the inside to the outsideor one direction from the outside to the inside regardless of theinformation layer having the information pits which are subjected totracking. In a case where a structure is employed in which the lightbeam is moved from the inside to the outside, a reproduction method maybe employed in which administration information is detected in theinternal portion of either of the information layers and an access ismade to a desired information region including the portion between theinformation layers. Therefore, the foregoing structure is suitable to arecording medium in which high speed access must be permitted.

As a method of obtaining the second substrate 3 with which the foregoingstructure can be realized, the foregoing method may be employed in whichthe second master to which the master has been transferred again is usedto reverse the directions of the information pits. That is, transferenceof the surface of the master manufactured by the photoresist to thesecond master enables columns of information pits formed in the oppositedirections and in the opposite spiral directions to be obtained. Byadhesive-bonding the second substrate 3 and the first substrate 1through the separation layer 5, a recording medium having the samespiral direction when viewed from a light incidental position can beobtained.

In a case where the process for manufacturing the second master isomitted, the recording direction employed in the mastering process ischanged so that the first and second information layers 2 and 4 havingthe same spiral direction when viewed from the light incidental positionare obtained. That is, when the photoresist is exposed to light, thedirection in which the flat glass plate is made to be opposite to thatemployed when the master of the first substrate 1 is manufactured sothat the first and second information layers 2 and 4 having the samespiral direction when viewed from the light incidental position areobtained. Note that the information pits when viewed from the lightincidental position are made to be opposite to each other between thefirst and second information layers 2 and 4.

A second structure is arranged such that the projection and pit columnson the first and second substrates 1 and 3 run in opposite directionswhen viewed from the light incidental position. In the foregoing case,the direction in which the light beam is moved (in a direction from theinside to the outside or a direction from the outside to the inside) ismade to be opposite in accordance with the information layer having theinformation pits which have been subjected to tracking.

The foregoing structure is effective in a case where informationcontinued for a long time is treated. An example of a structure will nowbe described in which the light beam is moved from the inside portion tothe outside portion of the information pits on the first informationlayer 2 and the light beam is moved from the outside portion to theinside portion of the information pits on the second information layer4. After the light beam has reproduced final information in the outerportion of the first information layer 2, the light beam is moved to theouter portion of the second information layer 4 (that is, an opticalpickup makes an access to the information start point of the secondinformation layer 4 while keeping the same position). The light beamcontinuously starts reproduction of information at the outer portion ofthe second information layer 4. Since the foregoing informationreproducing method does not involve the optical pickup being moved whenthe light beam is moved from a layer to another layer, time loss caneffectively be prevented during the movement of the light beam. In acase where the recording pits are in a CLV mode (a constant linevelocity mode), the position of the optical pickup is not changed and,therefore, the change in the rotational speed can effectively beprevented.

As a method of mastering the second substrate 3 which is able to realizethe foregoing surface, a method may be available in which the positionat which recording of the signals starts is made to be opposite to thatemployed in the exposing process when the master of the first substrate1 is manufactured. In a case where recording of information of the firstsubstrate 1 starts at an inner position, a master exposed to light fromthe outer portion is used. Since the recording medium manufactured byusing the thus-obtained first and second substrates 1 and 3 comprisesinformation pits on the first and second substrates 1 and 3 which areformed in the opposite directions when viewed from the light incidentalposition, the tracking polarity must be switched between the informationlayers.

As another method of mastering the second substrate 3, a method may beemployed in which exposure of the outer portion is performed similar tothe foregoing case in a state where the direction in which the flatglass plate is made to be opposite to that employed when the master ofthe first substrate 1 is manufactured. Since the recording mediummanufactured by using the thus-obtained first and second substrates 1and 3 has the information pits on the first and second substrates 1 and3 which are formed in the same direction when viewed from the lightincidental position, the necessity of switching the tracking polaritybetween the information layers can be eliminated.

The foregoing structures having both reproduction only information layerand a recording and reproducing information layer will now be described,which are a reproduction only-recording and reproducing type structure(B) and a recording and reproducing-reproduction only structure (C).

When the foregoing structures (B) and (C) are subjected to a comparison,the foregoing structure (B) is advantageous in which the firstinformation layer is made to be the reproduction only information layerand the second information layer is made to be the reproducing andrecording information layer because the light absorption into the firstinformation layer can be reduced. In the case of the foregoing structure(C) in which the first information layer is the recording andreproducing type information layer, light absorption is required torecord information on the information layer. In the foregoing case,diffraction of transmitted light due to the recording mark is generatedwhen signals are recorded on the first information layer 2. Thus, thequantity of light which is able to reach the second information layer 4is reduced.

FIG. 3 is a cross sectional view showing an example of a structure ofthe optical information recording medium having the foregoing structure(B) of the reproduction only-recording and reproducing type informationlayers. As shown in FIG. 3, information pits 38 corresponding toinformation signals are formed on one surface of a first substrate 31having a thickness of d31. Moreover, on one surface of the firstsubstrate 31, there is formed a first information layer 32 having apredetermined transmissivity, a predetermined reflectance and athickness of d32. On one surface of a second substrate 33 having athickness of d33, there are formed tracking guide grooves 39 or samplepits. On one surface of the second substrate 33, there is formed asecond information layer 34 formed by a thin film, the opticalcharacteristic of which is changed when irradiated with the light beam 7and which has a thickness of d34. Between the first information layer 32and the second information layer 34, there is formed a transparentseparation layer 35 for positioning the first information layer 32 andthe second information layer 34 to be apart from each other for apredetermined distance d35.

The first information layer 32 has a predetermined transmissivity withrespect to the light beam 7 in order to allow light having apredetermined intensity to reach the second information layer 34. Thetemperature of a portion of the second information layer 34 that hasbeen irradiated with the light beam 7, the intensity of which has beenintensified, is raised. As a result, the optical characteristic of thesecond information layer 34 is changed so that information is recordedon the second information layer 34. Thus, the second information layer34 has a structure capable of satisfying both high coefficient ofabsorption with respect to the light beam 7 and great optical change,that is, a high efficiency in reproducing recorded signals.

Since the first information layer 32 is a reproduction only informationlayer, the first substrate 31 has, on the surface thereof, informationpits 38 corresponding to information signals. Since the secondinformation layer 34 is a recording and reproducing type informationlayer, the second substrate 33 has, on the surface thereof, guidegrooves comprising convex and concave portions for controlling trackingfor locating the light beam when information is recorded or sample pits(not shown) consisting of a pair of a projection and a pit which areshifted in the tracking direction to correspond to the sample-servo typetracking operation. In a case where the foregoing substrates are in theform of a disk, it is preferable that the information pits, the guidegrooves or the sample pits be formed into a spiral that is formed in thesame direction when viewed from a position upon which the light beam 7is made incident.

A recording medium having the information layers of the foregoingstructure (D) consisting of the recording and reproducing informationlayer and the recording and reproducing information layer will now bedescribed with reference to FIG. 4. As shown in FIG. 4, a firstsubstrate 41 having a thickness of d41 has one surface on which trackingguide grooves 48 or sample pits are formed. The first substrate 41 hasone surface having a first information layer 42 which has apredetermined transmissivity and a predetermined reflectance, theoptical characteristic of which changes when irradiated with the lightbeam 7, and which has a thickness of d42. On one surface of the secondsubstrate 43 having a thickness of d43, there are formed tracking guidegrooves 49 or sample pits. On one surface of the second substrate 43,there is formed a second information layer 44 formed by a thin film, theoptical characteristic of which is changed when irradiated with thelight beam 7 and which has a thickness of d44. Between the firstinformation layer 42 and the second information layer 44, there isformed a transparent separation layer 45 for positioning the firstinformation layer 42 and the second information layer 44 apart from eachother for a predetermined distance d45.

Also in the foregoing case, it is effective when the foregoing structureon the surface of the substrate is adopted to information pits for thereproduction only information layer. In particular, the address pit tobe formed on the surface of the substrate together with the guidegrooves or the sample pits so as to administer the recording medium isable to employ all of the methods adapted to the foregoing informationpits for only reproducing information.

The information layers for recording and reproducing information is ableto employ the information layers employed in the case of the foregoingstructure (B) consisting of the reproduction only information layer andthe recording and reproducing type information layer. In the foregoingcase, the first information layer 42 must have characteristics such thatit absorbs the light beam 7 in a predetermined quantity, its state ischanged because its temperature is raised, the changed state can bedetected as change in the reflected light, and it permits light to passthrough in a predetermined quantity in order to enable the secondinformation layer 44 to record and reproduce information. Moreover, thefirst information layer 42 must maintain the characteristic that lightpenetration is permitted even after information has been recorded. Asdescribed above, the first information layer 42 must be designed so asto be formed into a thin film with which high quality signals can beobtained and a required transmissivity realized before and afterinformation has been recorded can be obtained.

The thin film forming the first information layer 42 has, like thephase-change material, an optical constant that is changed, the changedstate being detected as change in the reflectance. In a case where thesecond information layer 44 is irradiated with the light beam in thestate where information has been recorded on the first information layer42, a portion of light, which has passed through the first informationlayer 42, is diffracted. Residual beams are converged onto the secondinformation layer 44. Therefore, the intensity of the light beam 7 mustbe set to a high level as compared with that to be applied to thereproduction only information layer.

In view of satisfactorily reproducing signals, the thickness d45 of theseparation layer 45 must be larger than the focal depth, and preferablyat least, 5 times the focal depth. Thus, the number of recording marksincluded in the light beam when the first information layer 42 ispenetrated by the light beam is made to be 25 or more which is thesquare of 5. As a result, an influence of crosstalk or the like can beinhibited.

In the case where the first information layer 42 is formed into amagneto-optic recording type thin film, the direction of magnetizationof which is changed, transmitted light does not diffract. Since thenecessity of considering the change in the transmitted light before andafter information has been recorded can be eliminated, an advantage canbe realized. However, the first information layer 42 must absorb lightin a predetermined quantity in order to record information. Therefore,it is preferable that the quantity of light to be applied be set to belarger than that set in the case of the reproduction only informationlayer.

The optical recording medium has a characteristic that a reproductiononly medium and a recording and reproducing medium are able to coexistwith each other. Moreover, a so-called partial ROM disk can bemanufactured in which the same medium surface has a reproduction onlyregion formed in the inner portion thereof and a recording andreproducing region formed in the outer portion thereof.

A recording medium having four information layers will now be described,which is an improvement of the recording medium having the twosubstrates having convex and concave portions on the surface thereof andadhesive-bonded to each other. This recording medium is described withreference to FIG. 5.

As shown in FIG. 5, information pits corresponding to informationsignals or guide grooves for controlling tracking of the light beam orsample pits are formed on one surface of a first substrate 58. Moreover,a first information layer 59 for transmitting a portion of the lightbeam 7 made incident upon the first substrate 58 and having apredetermined reflectance is formed on one surface of the firstsubstrate 58. On one surface of the second substrate 60, there areformed information pits corresponding to information signals or guidegrooves for controlling tracking of the light beam or sample pits. Onone surface of the second substrate 60, a second information layer 61 isformed which has a reflectance higher than that of the first informationlayer 59. The first information layer 59 and the second informationlayer 61 are positioned opposite to each other. Between the firstinformation layer 59 and the second information layer 61, there isformed at least a first separation layer 62. A third substrate 63 havinga similar thickness to that of the first substrate 58 has a surface onwhich information pits corresponding to information signals or guidegrooves for controlling tracking of the light beam or sample pits areformed. On one surface of the third substrate 63, there is formed athird information layer 64 which transmits a portion of the light beammade incident on the third substrate 63 and which has a predeterminedreflectance. A fourth substrate 65 having substantially the samethickness as that of the second substrate 60 has a surface on whichinformation pits corresponding to information signals or guide groovesfor controlling tracking of the light beam or sample pits are formed.Moreover, on one surface of the fourth substrate 65, there is formed afourth information layer 66 having a reflectance higher than that of thethird information layer 64. The third information layer 64 and thefourth information layer 66 are formed opposite to each other. Betweenthe third information layer 64 and the fourth information layer 66,there is formed a transparent second separation layer 67 in the form ofat least one layer, the second separation layer 67 having a thicknesssimilar to that of the first separation layer 62. The second substrate60 and the fourth substrate 65 are formed opposite to each other.Between the second substrate 60 and the fourth substrate 65 there isformed an adhesive layer 68.

Since the recording medium having the foregoing structure has avertically symmetric structure with respect to the adhesive layer 68, itcan be said that the foregoing structure is stable even if change, suchas change in the environment temperature, occurs.

A method and apparatus for manufacturing the recording medium havingplural information layers will now be described.

If the interval between two information layer is too short in the casewhere each of the two information layers is reproduced, reflected lightfrom another information layer or light which has transmitted throughthe same generates crosstalk so that an influence of change in theamplitude of the reproduced signal or that of distortion of the servosignal occurs. If the interval between the two information layers is toolong, a aberration is generated in the light converged spot in either ofthe information layers. To inhibit the foregoing influences, theinterval between the two information layers must be made to be constant.In order to achieve this, a separation layer exhibiting excellentaccuracy in the thickness thereof is required. Moreover, the twosubstrates must be adhesive-bonded to each other in such a manner thatthe central positions of the information pits or sample pits or theguide grooves coincide with each other. Note that the foregoingdescription is applied only to the case where the recording medium is inthe form of a disc shape and information is recorded when the recordingmedium is rotated. In a case of a recording medium having the twoinformation layers, the tracking control must be performed whileconsidering the allowable eccentricity in the case of a recording mediumhaving one information layer and a second eccentricity occurring due todeviation in the central positions between the two information layers.The present invention is intended to prevent the second eccentricity soas to compensate the tracking servo performed by the recording medium ofthe foregoing type.

In view of the foregoing, a method of manufacturing the two substrateswill now be described. Note that the first substrate is formed by theconventional method. That is, the first substrate is obtained by thesteps of manufacturing a master in the mastering process and performinginjection molding in a mold. The second substrate may be manufactured bythe same process as that for manufacturing the first substrate or amethod in which a second master is formed by again duplicating themaster in order to reverse the information pits. The injection moldingprocess may be performed by a process similar to that employed tomanufacture the first substrate. Note that the injection molding machinefor use in this embodiment has the master comprising the spiral orconcentric information pits and guide grooves, the center of whichaccurately coincide with the center of a central opening forming machinefor forming the central openings of the first and second substrates. Byusing the injection molding machine of the foregoing type, first andsecond substrates can be obtained in which center of the central openingand that of the information pits or the guide grooves do not deviateconsiderably.

An adhesive-bonding apparatus for adhesive-bonding the first and secondsubstrates with a separation layer having a predetermined thickness willnow be described. FIG. 6 is a schematic cross sectional view of theadhesive-bonding apparatus. As shown in FIG. 6, the adhesive-bondingapparatus comprises an upper-portion support section 61 for supportingthe second substrate 3; a lower-portion support section 62 including alight source 81 for supporting the first substrate 1 and hardening theseparation layer; an elevation section 63 for elevating theupper-portion support section 61; a resin-applying nozzle 64 forapplying a resin material 80 for forming the separation layer to thefirst substrate 1; and a base 65 for supporting the overall system.

The upper-portion support section 61 comprises a substrate supportsection 66a which is in contact with the flat surface of the secondsubstrate 3 to secure the second substrate 3; an upper base section 66bfor establishing the connection between the substrate support section66a and the elevation section 63; an upper shaft 84 downwards projectingover the central portion of the substrate support section 66a and havinga tapered recess at a leading end thereof to correct the positionalrelationship of the lower-portion support section 62; and a firstinner-portion guide section 68 having a tapered portion for adjustingthe position of the second substrate 3. Around the upper shaft 84, aspring 69 for downwards pressing the first inner-portion guide section68 with predetermined force is disposed. On a region of the substratesupport section 66a which is in contact with the second substrate 3,there is formed a suction opening 71 for securing the second substrate 3by vacuum action, with air being discharged to an external pump throughan evacuation port 70 formed in the substrate support section 66a.

On the other hand, the lower-portion support section 62 comprises a basesupport section 72 for supporting the first substrate 1; a light-sourcebox 73 for securing the base support section 72 to the base 65 andaccommodating a light source 81; a lower shaft section 74 having atapered projection at the leading end thereof to face the recess of theupper shaft 84; and a second inner-portion guide section 75 foradjusting the position of the first substrate 1 by the tapered portionthereof. Around the lower shaft section 74, there is disposed a spring46 for upwardly pressing the second inner-portion guide section 75 withpredetermined force.

The light source 81 hardens a resin material 80 for forming theseparation layer and is disposed in the bottom portion of thelight-source box 73 and immediately below the base support section 72.Therefore, the base support section 72 is made of a material whichpermits light emitted by the light source 81 to pass through, forexample, glass or a resin. Moreover, a portion of the base supportsection 72, which is in contact with the first substrate 1, has anadsorption opening 78 for securing the first substrate 1 by vacuumadsorption so that air is discharged to an external pump through anevacuation opening 77 formed in the light-source box 73. In a portion ofthe light-source box 73 outward from the first substrate 1 and oppositeto the substrate support section 66a, there is disposed a spacer 79 formaintaining the thickness d5 of the separation layer.

The resin-applying nozzle 64 extrudes resin material 80 supplied from anexternal resin reservoir tank through its leading end thereof so as toapply the resin material 80 to the upper surface of the firstsubstrate 1. The leading end of the resin-applying nozzle 64 is arrangedto move on a circle, the radius of which is about 2/3 of the radius ofthe first substrate 1 relative to the central axis of the firstsubstrate 1. Note that the resin-applying nozzle 64 is removed from theregion on the lower-portion support section 62 if it does not apply theresin material 80 to the upper surface of the first substrate 1.

FIG. 7 shows a structure of the apparatus according to this embodimentfor obtaining a separation layer having a predetermined thickness. FIG.7 is a partial cross sectional view showing the state of theadhesive-bonding apparatus shown in FIG. 6 in which the upper-portionsupport section 61 has been moved downwards and the first substrate 1and the second substrate 3 have been adhesive-bonded to each otherthrough the separation layer 5. To obtain the thickness d5 of theseparation layer 5, a spacer 79 having a thickness of d79 is disposedadjacent to the outer ends of the first and second substrates 1 and 3.The thickness d79 of the spacer 79 must satisfy the following equation:

    d79=d1+d3+d5                                               (7)

where d1 is the thickness of the first substrate 1 and d3 is thethickness of the second substrate 3.

To provide the separation layer 5 having the thickness of d5 at theinner portion of the first and second substrates 1 and 3, the lengthsd67 and d74 of the upper shaft 84 and the lower shaft section 74 mustsatisfy the following equation, assuming that the thickness of thesubstrate support section 66a is d65 and the thickness of thelight-source box 73 is d73:

    d67+d74=d65+d73+d1+d3+d5                                   (8)

By improving the accuracy of each section, a separation layer 5 in whichirregularity in the thickness is inhibited from the inner portion to theouter portion can be obtained.

Referring to FIGS. 6 and 8, a structure for performing anadhesive-bonding operation with which eccentricity is inhibited betweenthe first and second information layers will now be described. Thecenters of the upper-portion support section 61 and the lower-portionsupport section 62 are adjusted by a tapered section 82 of the recess inthe upper shaft 84 and a tapered section 83 of the recess in the lowershaft 74. When the upper-portion support section 61 has been moveddownwards by the elevation section 63, the tapered section 82 of therecess in the upper shaft 84 and the tapered section 83 of the recess inthe lower shaft 74 correct the centers of the upper-portion supportsection 61 and the lower-portion support section 62. When the horizontalplanar portions of the two shafts 67 and 74 are brought into contactwith each other at the lowermost position, the deviation of the centersof the upper-portion support section 61 and the lower-portion supportsection 62 is on the order of several μm or less, which is determined bythe machining accuracy of the two shafts 67 and 74.

To keep a constant positional relationship between the first and secondsubstrates 1 and 3 and the upper shaft 84 and the lower shaft section74, the first and second inner-portion guide sections 68 and 75 areformed, which are in contact with the columnar portions of the twoshafts 84 and 74 and which have the same central axis as those of theshafts 84 and 74. The first and second inner-portion guide sections 68and 75 are formed into tapered shapes having leading ends, the diametersD68 and D75 of which respectively are smaller than the diameters D3 andD1 of the openings in substrates 3 and 1, and other ends, the diametersD69 and D76 of which respectively are larger than the diameters D3 andD1 of the substrates 3 and 1. Each of the first and second inner-portionguide sections 68 and 75 is able to move in the vertical direction ofthe upper shaft 84 and the lower shaft section 74. The springs 69 and 76disposed around the two shafts 84 and 74 press the second substrate 3downwards and pushes the first substrate 1 upwardly. As described above,the central openings of the first and second substrates 1 and 3 havingthe diameters D1 and D3 are formed with excellent positional accuracy bythe injection molding machine. The central openings of the substrates 3and 1 are received by the tapered sections of the first and secondinner-portion guide sections 68 and 75. The substrates 3 and 1 areadsorbed through the adsorption openings 71 and 78 formed in thesubstrate support sections 66a and 72. In a state where the taperedsections of the first and second inner-portion guide sections 68 and 75are in contact with the central openings of the substrates 3 and 1, thesubstrates 3 and 1 are secured to the surface of the substrate supportsections 66a and 72. As a result, the central axis of the informationlayer of the second substrate 3 and the central axis of theupper-portion support section 61 coincide with each other with anaccuracy that is essentially the mechanical accuracy. Similarly, thecentral axis of the information layer of the first substrate 1 and thecentral axis of the lower-portion support section 62 coincide with eachother with an accuracy that is essencially the mechanical accuracy. Bymoving the upper-portion support section 61 downwards in the foregoingstate, the tapered section 82 of the recess in the upper shaft 84 andthe tapered section 83 of the recess in the lower shaft 74 make thecenter axis of the information layer of the first substrate 1 and thecenter axis of the information layer of the second substrate 3 coincidewith each other.

By using the adhesive-bonding apparatus having the foregoing structure,a recording medium can be obtained in which deviation of the circulararc of the information pits, sample pits or guide grooves formed on thesurfaces of the two information layers can be inhibited.

A method of manufacturing the optical information recording mediumhaving the two information layers in such a manner that theadhesive-bonding apparatus shown in FIG. 6 is used will now be describedwith reference to sequence charts shown in FIG. 9.

Initially, as shown in FIG. 9(a), a first information layer 2 forpermitting a penetration of a portion of light beams and having apredetermined reflectance is formed, by sputtering or an evaporationmethod, on a first substrate 1 having on its surface, information pitscorresponding to information signals, or guide groov es for controllingtrack ing of the light beam or sample pits. As shown in FIG. 9(b), asecond information layer 4 having a reflectance higher than that of thefirst information layer 2 is formed, by sputtering or an evaporationmethod, on a second substrate 3 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample pits, and a thickness substantiallythe same as that of the first substrate 1. Then, the fir st substrate 1is secured to a base support section 72. Then, a resin-applying nozzle64 is used to apply a photosetting resin material 80 to the uppersurface of the first information layer 2, as shown in FIG. 9(c). Then,the second substrate 3 is placed on a substrate support section 66a, andthen an elevation section 63 is operated to move an upper-portionsupport section 61 downwards to come into contact with a spacer 79.Then, the second substrate 3 and the first substrate 1 areadhesive-bonded to each other with a separation layer 5 at an interval,which is the thicknesses d5 of the separation layer 5 (see FIG. 9(d)).Then, as shown in FIG. 9(e), the outer surface of the first substrate 1is irradiated with light beams 73 emitted from a light source 81 so thatthe resin material 80 is hardened so that the separation layer 5 isformed. As a result of the foregoing process, a recording medium havingthe two information layers 2 and 4 can be obtained.

By employing the foregoing method, the process for removing the masteris not required to obtain a recording medium having a double layerstructure such that substrates having information pit surfaces formedpreviously are simply adhesive-bonded to each other. As a result, themanufacturing yield can be improved.

Although the structure has been described in which the resin material 80is applied to the upper surface of the first information layer 2, theresin material 80 may be applied to the upper surface of the secondinformation layer 4.

A method of manufacturing a light recording medium having fourinformation layers in such a manner that adhesive-bonding apparatusshown in FIG. 6 is used will now be described with reference to thesequence charts shown in FIG. 10.

As shown in FIG. 10(a), initially, a first information layer 59 forpermitting a penetration of a portion of light beams and having apredetermined reflectance is formed, by sputtering or an evaporationmethod, on a first substrate 59 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample pits.

As shown in FIG. 10(b), a second information layer 61 having areflectance higher than that of the first information layer 59 is formedon a second substrate 60 having on its surface, information pitscorresponding to information signals, or guide grooves for controllingtracking of the light beam or sample bits, and a thickness substantiallythe same as that of the first substrate 1.

Then, the first substrate 58 is secured to a base support section 72.Then, a resin-applying nozzle 64 is used to apply a photosetting resinmaterial 80 to the upper surface of the first information layer 59, asshown in FIG. 10(c). Then, the second substrate 60 is placed on asubstrate support section 66a of the adhesive-bonding apparatus, andthen an elevation section 63 is operated to move an upper-portionsupport section 61 downwards to come into contact with a spacer 79.Then, the second substrate 60 and the first substrate 58 areadhesive-bonded to each other with a separation layer 62 at an interval,which is the thicknesses d62 of the separation layer 62. Then, the outersurface of the first substrate 58 is irradiated with light beams 102emitted from a light source 81 so that the resin material 80 is hardenedso that the separation layer 62 is formed (see FIG. 10(d)). As a resultof the foregoing process, a first recording medium 101 having twoinformation layers on one side thereof can be obtained.

The foregoing method is different from the process shown in FIG. 9 inwhich the first and second substrates 1 and 3 having similar thicknessesare adhesive-bonded to each other, in that the thickness of the secondsubstrate 60 to be adhesive-bonded with the first substrate 58 is notlimited in this embodiment. In the foregoing case, the thickness of thespacer 79 and the lengths of the upper shaft 84 and the lower shaft 74must be changed.

As a result of a process (see FIGS. 10(e) to 10(h)) similar to thatshown in FIGS. 10(a) to 10(d), a second medium 103 having two layersformed on one surface thereof is obtained by adhesive-bonding the thirdsubstrate 63 and the fourth substrate 65. Referring to FIGS. 10(e) to10(h), reference numeral 64 represents a third information layer, 66represents a fourth information layer, 67 represents a second separationlayer, and 104 represents a light beam emitted from the light source 81.In the foregoing case, it is preferable that the thickness of the firstsubstrate 58 and that of the third substrate 63 be substantially thesame. Moreover, it is preferable that the thickness of the secondsubstrate 60 and that of the fourth substrate 65 be substantially thesame. The second and fourth substrates 60 and 65 may be formed by amethod other than the method employed to form the first and thirdsubstrates 58 and 63. To maintain the thin substrate serving as therecording medium, for example, a Photo-Polymerization method may beemployed, permitting the substrate to be made thinner.

Then, an adhesive-bonding apparatus similar to that shown in FIG. 6 isused to adhesive-bond the first medium 101 having two layers formed onone surface thereof and the second medium 103 having two layers formedon one surface thereof to each other. Initially, as shown in FIG. 10(i),a photosetting resin 105 is applied to the upper surface of the secondsubstrate 60 of the first medium 101 having two layers formed on onesurface thereof. Then, as shown in FIG. 10(j), the second substrate 60and the fourth substrate 65 are adhesive-bonded to each other, followedby irradiating the outer surface of the first substrate 58 with thelight beams 106 emitted from the light source 81 so that the resinmaterial 105 is set to form an adhesive layer 68. As shown in FIG.10(j), the lengths of the spacer 79, the upper shaft 84 and the lowershaft 74 must be changed to correspond to the overall length of therecording medium, the lengths of the first and second separation layers62 and 67 and the lengths of the first to fourth information layers 59,61, 64 and 66. When the exposure process shown in FIG. 10(j) isperformed, the light beams 106 emitted from the light source 81penetrate the first and second information layers 59 and 61, and thenreach the resin material 105. Therefore, the quantity of the light beams106 must be enlarged as compared with the exposure processes, forexample, as shown in FIG. 9(e), 10(d) or 10(h).

As the resin material 105 for use when the first and second mediums 101and 103 each having two layers formed on one surface thereof areadhesive-bonded to each other, a resin that absorbs light beams may beemployed. A resin other than the photosetting resin may be employed. Forexample, a heat setting resin, a hot melt adhesive agent or anotheradhesive agent may be employed. Therefore, the irradiation with lightbeams may be omitted from the process shown in FIG. 10(j).

By employing the foregoing method, the process for removing the masteris not required. By sequentially adhesive-bonding substrates each havingthe surface comprising the information pits formed previously, arecording medium having a four-layer structure can be obtained. As aresult, the manufacturing yield can be improved. The recording mediumhaving the four-layer structure can be obtained by repeating three timesthe adhesive-bonding process required to obtain the recording mediumhaving the double-layer structure. That is, the recording medium havingthe four-layer structure can be realized by using basically the samemanufacturing apparatus. Thus, it can be realized by a method similar tothat required to obtain the recording medium having the double-layerstructure.

A recording and reproducing apparatus for recording and reproducing onthe optical information recording medium according to the presentinvention, as manufactured by the foregoing method, will now bedescribed with reference to FIG. 11. As shown in FIG. 11, the recordingand reproducing apparatus according to this embodiment comprises anoptical disk 111 that is basically an optical information recordingmedium having a plurality of information layers; a spindle motor 112 forrotating the optical disk; an optical pickup section 113 for converginglight beams, such as laser beams, emitted from a light source 121; andfive circuit systems for controlling the spindle motor 112 and theoptical pickup section 113. A first circuit system is a light modulatingsystem 114 for operating the light source 121 of the optical pickupsection 113. A second circuit system is a control system 115 forcontrolling the operation of the light beams to cause light beamsemitted from the optical pickup section 113 to be converged onto theoptical disk 111 and tracking for causing the light beams to follow theinformation pits or the guide grooves. A third circuit system is asignal reproduction system 116 for reading information signals formed onthe optical disk 111. At least one of the foregoing three circuitsystems has two or more types of condition setting functions to set anoptimum condition to each of the information layers. A fourth circuitsystem is a layer selection system 117 for switching the condition ofthe three circuit systems in accordance with the information layer towhich the light beams are to be detected. A fifth circuit system is asystem control system 118 for controlling the timing of the four circuitsystems.

The present invention has a structure such that the layer selectionsystem 117 is used to reproduce recorded information to select theoptimum condition of the foregoing circuit systems to enable informationto be recorded onto a plurality of information layers while inhibitingerrors in information to be reproduced from the plural informationlayers.

When the information signals are reproduced from the optical disk 111,the system control system 118 controls the rotation control section 119to rotate the spindle motor 112 so as to rotate the optical disk 111 atconstant speed. A control signal indicating the reproduction state issupplied to a laser drive section 120 so that the electric currentflowing to the light source 121 is controlled in such a manner that theintensity of light beams to be emitted from the optical pickup section113 is set at a reproduction power value as instructed by the systemcontrol system 118. The light beams emitted from the light source 121pass through the optical system of the optical pickup section 113 and anobjective lens 122 disposed at the rear portion so that the light beamsare made to be converged beams with which the optical disk 111 isirradiated.

The light beams reflected by the optical disk 111 again pass through theobjective lens 122 and the optical system in the optical pickup section113 so as to be made incident upon a photodetector 123 having a lightreceiving surface which is divided into sections. The photodetector 123photoelectrically converts the incidental light beams to transmit asignal having the voltage corresponding to the change in the quantity oflight on each of the light receiving surfaces to the signal reproductionsystem 116. The signal transmitted from the photodetector 123 isamplified by a pre-amplifier 124 so that low frequency components in thesignal are used to control the position of the light beam.

A focus control section 126 uses a portion of the signal transmittedfrom each of the light receiving surfaces of the photodetector 123 toobtain a focus error signal and operate a voice coil 125 by using thefocus error signal. As a result, the objective lens 122 is controlled tomove slightly in the perpendicular direction with respect to the surfaceof the optical disk 111 so that the light beams are converged onto thesurface of the information layer of the optical disk 111. The systemcontrol system 118 transmits, to a layer selection system 117, a layerselection signal for appointing an information layer to be focused inresponse to control signal S03. The layer selection system 117 switchesthe operations of the light modulating system 114, the control system115 and the signal reproduction system 116 in accordance with theinformation layer. As a result, signals in any information layer on theoptical disk 111 can be reproduced.

A layer identification section 132 demodulates a layer identificationsignal from the signal transmitted from a binary-coding section 130 toidentify the information layer that is being focused. If the informationlayer that is being focused is not the subject information layer, afocus jumping circuit 133 sequentially shifts the focusing positionamong the information layers. The focus jumping circuit 133 superimposesthe pulse voltage for instantaneously moving a voice coil 125 in theperpendicular direction with respect to the optical disk 111 on theoutput signal from the focus control section 126. As a result, the lightbeam can be converged onto the subject information layer.

A tracking control section 127 obtains a tracking control signal from acombination of the other output signals from the photodetector 123 insuch a manner that the light beam follows the information pits or theguide grooves, and then slightly moves the voice coil 125 in a directionof the radii of the optical disk 111. When the information layer of areproduction only type is reproduced, a polarity inverter 128 switchesthe tracking polarity among the information layers in accordance with aninstruction issued from the layer selection system 117 so that a phasedifference method or a 3-beam method is employed to perform tracking insuch a manner that the light beams reproduce the information pits of theinformation layer. In a case where the information layer of a recordingand reproducing type is reproduced, the polarity inverter 128 switchesthe tracking polarity or the tracking method among the informationlayers in accordance with an instruction issued from the layer selectionsystem 117. Thus, the information pits of the information layer of therecording and reproducing type are reproduced by tracking by a push-pullmethod in a case where the information layer has guide grooves or by asample servo method in a case where the information layer is composed ofwobble pits. By switching the tracking method according to the type ofthe information layer, the recording density of both of the informationlayers can be raised.

A tracking jumping circuit 129 superimposes the pulse voltage forinstantaneously moving the voice coil 125 in the direction of the radialof the optical disk 111 on the output signal from the tracking controlsection 127. As a result, the light beam can be moved onto the surfaceof the subject track.

In accordance with the output from the tracking control section 127, thepolarity inverter 128 inverts the polarity thereof in accordance withthe direction of the information pits formed on the information layer,and whether the light beam is allowed to follow the land or groove ofthe guide groove.

In the case where the recording medium has been manufactured by usingthe first and second substrates 1 and 3 obtained by the same masteringprocess as shown in FIG. 2(a), the directions of the pits are invertedbetween the first and second information layers 2 and 4 when viewed froma position upon which the light beam 7 is made incident. In the case ofa double-layer medium of the foregoing type, the focusing position ismoved between the first and second information layers 2 and 4 by thefocus jumping circuit 133. Simultaneously, the polarity inverter 128switches the tracking polarity between the first and second informationlayers 2 and 4. As a result, the light beam can be moved instantaneouslyonto the information pit of the subject information layer. Therefore,the foregoing method is able to reduce the time required to make anaccess between the information layers when information is reproduced.

The binary-coding section 130 of the signal reproduction system 116 usesthe high frequency components of the signal supplied from thepre-amplifier 124 to make a comparison between the level of theforegoing signal and a reference level so as to convert the signal intoa binary-coded signal. Then, the decoder 131 decodes the binary-codedsignal in accordance with a predetermined signal format. As a result,the information signal is demodulated from the recording mark formed onthe optical disk 111. Then, demodulation information S02 is transmittedto an external unit in accordance with an instruction issued from thesystem control system 118.

If necessary, conditions for reproducing or recording the informationlayer formed in a specific region on the optical disk 111 aredemodulated by the layer identification section 132. The layeridentification section 132 also has a function capable of demodulatingthe shape of the information layer and the like as well as identifyingthe information layer. It is preferable that the foregoing informationbe recorded during the process for manufacturing the recording medium.The contents of the information include identification information foridentifying whether the information layer is a reproduction onlyinformation layer or a recording and reproducing information layer orinformation for correcting the differences in the characteristics amongthe information layers. That is, the contents are inform ation about theoptimum condition for irradiating each information layer with light, theoptimum condition for performing the focus control or the trackingcontrol and the optimum condition when the reproduced signal isdemodulated.

In a case where information is recorded on a plurality of informationlayers, initially the system control system 118 causes the lightmodulating system 114 to receive recording information S01 composed ofinformation to be recorded at a predetermined timing. The lightmodulating system 114 initially causes an encoder 134 to convert therecording signal into a recording signal having a predetermined format,and then causes the laser drive section 120 to modulate the intensity oflight to be emitted from the light source 121 in accordance with thecondition of a waveform setter 135, which divides a pulse or sets achange in the intensity. Light having the modulated intensity isabsorbed into the recording layer on the optical disk 111. As a result,the reproducing mark can be formed on the recording layer on the opticaldisk 111 so that information is recorded.

Note that the waveform setter 135 has recording patterns which areoptimum for recording on the respective information layers and changesthe output therefrom in synchronization with the output from the layerselection system 117. The laser drive section 120 modulates theintensity of the light to be emitted from the light source 121 inaccordance with the modulated waveforms corresponding to the respectiveinformation layers.

By employing the foregoing structure, the information signals can bereproduced from the plural information layers under optimum conditions.Moreover, the information signals can be recorded onto the pluralinformation layers under optimum conditions and recorded information canbe reproduced.

The specific operations of the components of the recording andreproducing apparatus will now be described in detail.

FIG. 12 shows the structure of the optical pickup section. In thisembodiment, a knife edge method is employed as the focusing method and apush-pull method is employed as the tracking method.

As shown in FIG. 12, light emitted from the light source 121 passesthrough a collimator lens 140 so as to be formed into parallel beams,followed by being reflected by a beam splitter 141. Then, the reflectedbeams pass through a λ/4 plate 142 and the objective lens 122 so thatthe optical disk 111 is irradiated with the light beams. Light reflectedby the optical disk 111 passes through the objective lens 122, the λ/4plate 142 and the beam splitter 141, and then passes through a lens 143,and then a portion of the light beams is reflected and is made incidentupon the photodetector 145 having a plurality of light receivingsurfaces for performing the tracking operation. An output from each ofthe light receiving surfaces of the photodetector 145 is amplified bythe pre-amplifier 124 so that a tracking error signal is obtained from adifference signal.

On the other hand, light which is not reflected by the mirror 144 ismade incident upon the photodetector 146 having a plurality of lightreceiving surfaces for performing the focusing operation. An output fromeach of the light receiving surfaces of the photodetector 146 isamplified by the pre-amplifier 124 so that a focus error signal isobtained from the difference signal. Referring to FIG. 12, referencenumeral 113 represents the optical pickup and 120 represents the laserdrive section.

FIG. 13 shows a portion of a focus control section for performing thefocus control in accordance with the output from the photodetector.Although a usual knife edge method employs a photodetector having alight receiving surface divided into two sections, this embodiment hasan arrangement such that the photodetector 146 has a light receivingsurface divided into at least four sections, as shown in FIG. 13. Thereason for this is that if a photodetector having the light receivingsurface divided into two sections is used, then an operation ofobtaining a servo signal from a subject information layer encounters theproblem of the servo signal being distorted because a portion of lightreflected by other information layers is made incident upon thephotodetector. Although reduction in the area of the light receivingportion of the photodetector enables the distortion of the servo signalto be inhibited, another problem arises in that a range for pullingfocusing is excessively limited.

Accordingly, this embodiment has a structure such that the lightreceiving surface of the photodetector 146 is divided into at least foursections. Moreover, a method is employed in which the focus detectionregion is switched between a focus pulling stage and the servo operationstage. As shown in FIG. 13, the photodetector 146 has the lightreceiving surface divided into light receiving surfaces 146a, 146b, 146cand 146d. An output from each of the light receiving surfaces 146a,146b, 146c and 146d of the photodetector 146 is amplified by each ofamplifiers 147a, 147b, 147c and 147d so that two types of focus errorsignals 148s and 149s are obtained by difference amplifiers 148 and 149.Then, a switching unit 150 selects either of the focus error signal 148sor 149s. The selected focus error signal 148s (or 149s) passes through afocus operation circuit 151 and the focus jumping circuit (see FIG. 11),and then operates the optical pickup section 113 (see FIG. 11).

The focus error signal will now be described with reference to FIG. 14in both of a case where the photodetector for the focusing operation isdivided into two sections and a case where the same is divided into foursections. The axis of abscissa stands for focus-directional positions inwhich the positions of the two information layers are indicated by L1and L2. FIG. 14(a) shows a case where the light receiving surface isdivided into two sections and the light receiving surface is large. FIG.14(b) shows a case where the light receiving surface is divided into twosections and the light receiving surface is small. FIG. 14(c) shows acase where the light receiving surface is divided into four sections andthe outer light receiving surfaces 146a and 146d are used. FIG. 14(d)shows a case where the light receiving surface is divided into foursections and the inner light receiving surfaces 146b and 146c are used.In the case shown in FIG. 14(a) where the light receiving surface isdivided into two sections and a photodetector 151 having a large lightreceiving surface is employed, and in a case where a focal point existsnear focus beam F1 from an information layer, reflected light F2 fromanother information layer is made incident upon a portion of the lightreceiving surface. Therefore, the focus error signal is distorted and,therefore, a focal-point positional error dF occurs. In the case shownin FIG. 14(b) where the photodetector 152 is used which has the lightreceiving surface divided into two sections and having a small lightreceiving surface area, receipt of leaked light beam from anotherinformation layer can be prevented. Moreover, S-figure curves of the twofocus error signals appear at the positions corresponding to thepositions L1 and L2 of the information layers so that the servooperation is enabled. However, the realized focus pull-in range M2 issmaller than pull-in range M1 in the case shown in FIG. 14(a) in whichthe light receiving surface has a large area. Thus, the operationbecomes unstable if the recording medium is warped or has anirregularity on its surface.

To overcome the foregoing problems, this embodiment employs thephotodetector 146 having a light receiving surface divided into foursections. Note that the light receiving surface is formed in such amanner that the reflected beam F1 from either information layer ispositioned at substantially the central position of a division linebetween the light receiving surfaces 146a and 146b and the reflectedbeam F2 from another information layer is positioned at substantiallythe central position of a division line between the light receivingsurfaces 146c and 146d. In a case where the distance between the centersof the reflected beams F1 and F2 on the light receiving surface is Lfand the size of each of the spots of the reflected beams F1 and F2 onthe light receiving surface is Ld, width 146w of the outer lightreceiving surfaces 146a and 146d is set to be larger than Lf and smallerthan Lf+Ld. FIG. 14(c) shows a focus error signal in a case where thelight receiving surface is divided into four sections and the outerlight receiving surfaces 146a and 146b are used. Since the lightreceiving surfaces are apart from the reflected light beams F1 and F2from the two information layers, an S-figure curve appears, similar tothe case where one information layer is present. If a servo operation isperformed in response to the foregoing signal, the focusing position islocated between the position L1 of one of the information layers and theposition L2 of the other information layer. In the foregoing case, alarge range M3 can be obtained as the range in which the focusing signalis found. FIG. 14(d) shows a focus error signal in a case where thelight receiving surface is divided into four sections and inner lightreceiving surfaces 146b and 146c are used. The focus error signal issimilar to that of the case shown in FIG. 14(b) in which the lightreceiving surface is divided into two sections and the light receivingsurface has a small area.

The photodetector 146 having the light receiving surface divided intofour sections according to this embodiment is able to realize a largefocus pull-in range and stable focusing with respect to the twoinformation layers by switching the focus error signal in the case shownin FIG. 14C and the focus error signal shown in FIG. 14(d).

When focusing is pulled, the switching unit 150 selects a differencesignal 149s from the outer light receiving surfaces 146a and 146d of thephotodetector 146 so that the focus operating circuit 151 starts thefocusing operation. In the foregoing state, the focusing point ispositioned between the two information layers.

When completion of the operation for pulling focusing has been confirmedby the focus operating circuit 151, the focus operating circuit 151transmits a focus operation completion signal 151s to the switching unit150. In response to the focus operation completion signal 151s, theswitching unit 150 selects the difference signal 148s which is any ofoutputs from the inner light receiving surfaces 146b and 146c of thephotodetector 146 so that either of the information layers is subjectedto focusing. Then, the tracking operation is performed in apredetermined region to determine whether or not the subject informationlayer has been subjected to focusing. If an information layer which isnot the intended subject has been subjected to focusing, the focusjumping circuit 133 moves the focusing position to the subjectinformation layer. During the foregoing process, the switching unit 150does not perform the switching operation.

Although the description has been directed to a structure which employsthe knife edge method as the focusing method, the focusing method is notlimited to this. For example, an astigmatism method may be employed. Ifthe astigmatism method is employed, a cylindrical lens is disposed atthe position of the mirror 144 shown in FIG. 12 and a photodetector 154having a light receiving surface divided into 8 sections as shown inFIG. 15 is disposed near the photodetector 146. In the case where theastigmatism method is employed, outputs from the light receivingsurfaces 154a, 154b, 154c and 154d in the vicinity of the photodetector146 are used similar to the knife edge method. After the focus pull-inoperation has been completed, outputs from the light receiving surfaces154e, 154f, 154g and 154h in the central portion of the photodetector146 are used so that a focus error signal is obtained.

By employing the foregoing structure, a stable servo operation for eachof plural information layers can be performed while maintaining thefocus pull-in performance similar to that obtainable from theconventional structure.

The quality of a recording medium having plural information layersdepends upon the irregularity of the shapes of the information pits orguide grooves. The quality of the recording and reproducing apparatusdepends upon the distortion of the intensity distribution of the lightbeams or dispersion of the sensitivity of the photodetector or the like.Therefore, error voltage is generated in the focus error signal or thetracking error signal due to interference between the information layersor change in the thickness of the separation layer when the servooperation is performed.

In order to correct an error in the focus control signal or the trackingcontrol signal, the focus control section or the tracking controlsection is offset-adjusted in synchronization with the setting of thelayer selection system 117 (see FIG. 11). For example, a fine offset isadded to the focus control signal so that focus deviation generatedbetween the layers is corrected. Also a fine offset is added to thetracking control signal so that tracking deviation is corrected. Thus,an optimum light convergent state can be realized in each of theinformation layers.

FIG. 16 shows the focus control section i n detail. As shown in FIG. 16,a focus error signal 160s is produced by a focus error detection circuit160, obtained from a signal in the output signal 124s from thepre-amplifier 124 (see FIG. 12) and relating to the focus control. Thus,a focus control signal 126s can be obtained by a focus operation circuit162 through an offset compensation circuit 161. The focus control signal126s is transmitted to the optical pickup section 113 (see FIG. 11) sothat the voice coil 125 (see FIG. 11) is operated and the focus controlis performed.

The offset compensation circuit 161 has a structure capable of setting aplurality of offset levels in response to a signal supplied from theoutside. An offset setting unit for setting the offset to be supplied tothe offset compensation circuit 161 comprises an offset setting unit 163for setting an offset when focusing of the first information layer 2 hasbeen performed; and an offset setting unit 164 for setting an offsetwhen focusing of the second information layer 4 has been performed. Anoffset selector 165 responds to an output 117s from the layer selectionsystem 117 (see FIG. 11) to transmit the offset value of either of theoffset setting unit 163 or the offset setting unit 164.

On the other hand, the focus operation circuit 162 receives a signal161s transmitted from the offset compensation circuit 161 and transmitsa focus control signal 126s for making the signal 161s to be zero so asto operate the voice coil 125. The gain setting unit for setting thegain of a circuit when the focusing operation is performed comprises again setting unit 166 for setting the gain in the case of the firstinformation layer 2; and a gain setting unit 167 for setting the gain inthe case of the second information layer 4. A gain selection unit 168responds to an output 117s from the layer selection system 117 totransmit a signal from the gain setting unit 166 or the gain settingunit 167. By employing the foregoing structure, an optimum focusingstate can be set with respect to the two information layers.

As for the tracking control, the setting of an optimum state between theinformation layers enables reproduction or recording and reproducing tobe performed more satisfactorily. FIG. 17 shows the tracking controlsection in detail. As shown in FIG. 17, a tracking error signal 170s canbe obtained by a tracking error detection circuit 170 from a signalrelating to the tracking control in an output signal 124s from thepre-amplifier 124. A tracking control signal 127s can be obtained by atracking operation circuit 172 through an offset compensation circuit171. The tracking control signal 127s passes through the polarityinverter 128 (see FIG. 11) and is supplied to the optical pickup section113. Thus, the voice coil 125 is operated so that the tracking controlis performed.

The offset compensation circuit 171 is structured to be capable ofsetting a plurality of offset levels in response to a signal suppliedfrom the outside. The offset setting unit for setting offset to besupplied to the offset compensation circuit 171 comprises an offsetsetting unit 173 for setting the offset when focusing of the firstinformation layer 2 has been performed; and an offset setting unit 174for setting the offset when focusing of the second information layer 4has been performed. The offset selector 175 responds to an output 117sfrom the layer selection system 117 (see FIG. 11) to transmit an offsetvalue of the offset setting unit 173 or the offset setting unit 174.

On the other hand, the tracking operation circuit 172 receives a signal171s transmitted from the offset compensation circuit 171 and transmitsa tracking control signal 127s with which the signal 171s is made to bezero so that the voice coil 125 is operated. The gain setting unit forsetting the gain when the tracking operation is performed comprises again setting unit 176 for setting the gain in the case of the firstinformation layer 2; and a gain setting unit 177 for setting the gain inthe case of the second information layer 4. A gain selector 178corresponds to an output 117s from the layer selection system 117 totransmit a signal from the gain setting unit 176 or the gain settingunit 177. By employing the foregoing structure, an optimum trackingstate with respect to the two information layers can be set.

Although this embodiment has a structure such that the information pitsformed on the first substrate 1 are in the convex form when viewed froma position at which the light beam is made incident, the structure isnot limited to this. The information pits may be formed as concave whenviewed from a position at which the light beam is made incident. In theforegoing case, the directions of the information pits on the secondsubstrate 3 are inverted so that a recording medium having a similareffect to that obtainable from this embodiment is obtained.

Specific structures of the information recording medium will now bedescribed.

(EXAMPLE 1)

A method of manufacturing the optical information recording medium shownin FIG. 1 and an operation of recording and reproducing the opticalinformation recording medium will now be described.

Polycarbonate resin was employed to form the first and second substrates1 and 3, and a mold having a surface comprising information pits wasused to perform injection molding so that the first and secondsubstrates 1 and 3 were manufactured. The first substrate 1 had adiameter of 120 mm and a thickness of 1.2 mm and comprised, on thesurface thereof, information pits arranged such that the shortest pitlength was 0.83 μm, the pit depth was 100 nm and the track pitch was 1.6μm. The information pits consisted of pit columns formed to conform tothe EFM code. On the first substrate 1, there was formed an Au layerhaving a thickness of 10 nm by a sputtering method so that the firstinformation layer 2 was formed.

The second substrate 3 had the same diameter and thickness as those ofthe first substrate 1, the second substrate 3 having information pits inthe same form as those of the first substrate 1. To make the directionsof the spirals of the first substrate 1 and the second substrate 3 to bethe same when viewed from a position upon which the light beam 7 wasmade incident after adhesive-bonding, the direction of the spiral of theprojection and pit column when viewed from the surfaces of theinformation pits of the second substrate 3 was inverted with respect tothat of the first substrate 1. An Au film was formed to have a thicknessof 100 nm by a sputtering method so that the second information layer 4was formed. The information pits of the first and second substrates 1and 3 were formed into concave when viewed from a position at which thepits exist.

The first substrate 1 was secured to the substrate support section 72 ofthe adhesive-bonding apparatus shown in FIG. 6, followed by using theresin applying nozzle 64 to apply the acrylic type ultraviolet curingresin material 80 to the upper surface of the first information layer 2.The second substrate 3 was placed on the substrate support section 66aof the adhesive-bonding apparatus. Then, the upper-portion supportsection 61 was moved downwards by the elevation section 63 to bring thesame into contact with the spacer 79. While pressing the secondsubstrate 3 from an upper position with a load of 5 kg, it wasirradiated with light emitted from the light source (the ultravioletlamp) 81. Thus, the resin material 80 was hardened so that theseparation layer 5 having a thickness of d5 was formed between the firstinformation layer 2 and the second information layer 4.

Before the adhesive-bonding was performed, the thickness of the innerportion, intermediate portion and the outer portion of each substratewas previously measured to calculate the differences from those afteradhesive-bonding. Thus, the thickness d5 of the separation layer 5 wasobtained. As a result, the mean value of the thickness of the separationlayer 5 was 65 μm with an accuracy of ±8 μm or less at each measurementposition. The reflectance of the first information layer 2 at awavelength of 780 nm was 27.5% and that of the second information layer4 at a wavelength of 780 nm was 91.6%. The level of eccentricity betweenthe information layers was 40 μm.

Information was reproduced from the foregoing recording medium with anoptical system having a light source for emitting light having awavelength of 780 nm and an objective lens adaptable to an optimumthickness of the base of 1.2 mm and having a numerical aperture (NA) of0.5. A knife edge method was employed to perform focusing, while apush-pull method was employed to perform tracking. Focusing wasperformed by using the photodetector 146 shown in FIG. 13 and having alight receiving surface divided into four sections in such a manner thatthe light receiving surface of the photodetector 146 for obtaining afocus error signal was switched between the pull-in operation and theservo operation. Reproducing light had power of 1 mW when the signal wasreproduced. As a result, it was confirmed that stable focusingoperations were performed with respect to the first and secondinformation layers 2 and 4 and focus jumping was stably performed. Notethat the polarity of the tracking signal was switched between theinformation layers. An excellent eye pattern was observed in theobtained reproduced signal from both of the first and second informationlayers 2 and 4. Jitters of both signals were measured, thus resulting inexcellent values to be obtained such that the standard deviation withrespect to the width of the detection window was 8.4% in the case of thefirst information layer 2 and 8.7% in the case of the second informationlayer 4.

Then, the obtained recording medium was allowed to stand in a hot andwet environment, the temperature of which was 80=and the relativehumidity was 80%, for 100 hours, followed by performing a similarexperiment. Then, the signal was similarly evaluated. As a result,change in the shape was inhibited, information could stably bereproduced, and an excellent result was obtained in measuring jitterswithout considerable change.

As a result, the method according to the present invention is aneffective method for manufacturing a recording medium having a pluralityof information layers.

(EXAMPLE 2)

A structure of a recording medium capable of forming information moredensely will now be described. Similarly to Example 1, polycarbonateresin was employed to form the first and second substrates 1 and 3, anda mold having a surface comprising information pits was used to performinjection molding so that the first and second substrates 1 and 3 weremanufactured. The first substrate 1 had a thickness of 0.58 mm andcomprised, on the surface thereof, information pits arranged such thatthe shortest pit length was 0.5 μm, the pit depth was 90 nm and thetrack pitch was 0.8 μm. On the first substrate 1, there was formed an Aulayer having a thickness of 11 nm by a sputtering method so that thefirst information layer 2 was formed.

The second substrate 3 had the same thickness as that of the firstsubstrate 1, the second substrate 3 having information pits in the sameform as those of the first substrate 1. To make the directions of thespirals of the first substrate 1 and the second substrate 3 to be thesame when viewed from a position near the light source afteradhesive-bonding, the direction of the spiral of the projection and pitcolumn when viewed from the surfaces of the information pits of thesecond substrate 3 was inverted with respect to that of the firstsubstrate 1. An Au film was formed to have a thickness of 100 nm by asputtering method so that the second information layer 4 was formed. Tomake the shape of the pit on the main reflecting surface after thesecond information layer 4 had been formed to be similar to that of thefirst substrate 1, the length of the shortest pit of the pits to beformed on the surface of the second substrate 3 was made to be 0.6 μm.However, the pitch of the pits and the track pitch were the same ofthose of the first substrate 1.

The first substrate 1 was secured to the substrate support section 72 ofthe adhesive-bonding apparatus shown in FIG. 6, followed by using theresin applying nozzle 64 to apply the acrylic type ultraviolet curingresin material 80 to the upper surface of the first information layer 2.The second substrate 3 was placed on the substrate support section 66aof the adhesive-bonding apparatus. Then, the upper-portion supportsection 61 was moved downwards by the elevation section 63 to bring thesame into contact with the spacer 79. While pressing the secondsubstrate 3 from an upper position with a load of 8 kg, it wasirradiated with light emitted from the light source (the ultravioletlamp) 81. Thus, the resin material 80 was hardened so that theseparation layer 5 having a thickness of d5 was formed between the firstinformation layer 2 and the second information layer 4.

Before the adhesive-bonding was performed, the thickness of the innerportion, intermediate portion and the outer portion of each substrateare previously measured to calculate the differences from those afteradhesive-bonding. Thus, the thickness d5 of the separation layer 5 wasobtained. As a result, the mean value of the thickness of the separationlayer 5 was 52 μm with an accuracy of ±5 μm or less at each measurementposition. The reflectance of the first information layer 2 at awavelength of 680 nm was 28.2% and that of the second information layer4 at a wavelength of 680 nm was 89.6%. The level of eccentricity betweenthe information layers was 35 μm.

Information was reproduced from the foregoing recording medium with anoptical system having a light source for emitting light having awavelength of 680 nm and an objective lens adaptable to an optimumthickness of the base of 0.6 mm and having a numerical aperture (NA) of0.6. A similar servo method to that employed in Example 1 was employed.As a result, it was confirmed that stable focusing operations wereperformed with respect to the first and second information layers 2 and4 and focus jumping between the information layers was stably performed.Note that the polarity of the tracking signal was switched between theinformation layers. An excellent eye pattern was observed in theobtained reproduced signal from both of the first and second informationlayers 2 and 4. Jitters of the both signals were measured, thusresulting in excellent values to be obtained such that the standarddeviation with respect to the width of the detection window was 7.6% inthe case of the first information layer 2 and 8.0% in the case of thesecond information layer 4.

Then, the obtained recording medium was allowed to stand in a hot andwet environment, the temperature of which was 80° C. and the relativehumidity was 80%, for 100 hours, followed by performing a similarexperiment. Then, the signal was similarly evaluated. As a result,change in the shape was inhibited, information could stably bereproduced, and an excellent result was obtained in measuring jitterswithout considerable change.

(EXAMPLE 3)

An example of a recording medium having four information layers as shownin FIG. 5 will now be described. Similarly to Example 1, polycarbonateresin was employed to form the first to fourth substrates 58, 60, 63 and65, and a mold having a surface comprising information pits was used toperform injection molding so that the first to fourth substrates 58, 60,63 and 65 were manufactured. Each of the first and third substrates hada thickness of 0.58 mm and comprised, on the surface thereof,information pits arranged such that the shortest pit length was 0.5 μm,the pit depth was 90 nm and the track pitch was 0.8 μm. On each of thefirst and third substrates 58 and 63, there was formed an Au layerhaving a thickness of 11 nm by a sputtering method so that the first andthird information layers 59 and 64 were formed.

Each of the second and fourth substrates 60 and 65 had a thickness of0.4 mm, which was smaller than that of each of the first and thirdsubstrates 58 and 63, in order to reduce the overall thickness of therecording medium after adhesive-bonding, each of the second and fourthsubstrates 60 and 65 having information pits having the same shapes asthose of the first and second substrates 58 and 63 on the surfacethereof. To make the directions of the spirals of the first and secondsubstrates 58 and 63 and the second and fourth substrates 60 and 65 tobe the same when viewed from a position near the light source afteradhesive-bonding, the direction of the spiral of the information pitcolumn of the second and fourth substrates 60 and 65 was inverted withrespect to that of the first and second substrates 58 and 63. On thesecond and fourth substrates 60 and 65, an Au film was formed to have athickness of 100 nm by a sputtering method so that the second and fourthinformation layers 61 and 66 were formed. To make the shapes of the pitson the main reflecting surfaces after the second and fourth informationlayers 61 and 66 had been formed to be similar to those of the first andthird substrates 58 and 63, the shortest length of the pit among thepits to be formed on the surfaces of the second and fourth substrates 60and 65 was made to be 0.6 μm.

The first substrate 58 was secured to the substrate support section 72of the adhesive-bonding apparatus shown in FIG. 6. The resin applyingnozzle 64 was used to apply the acrylic ultraviolet curing resinmaterial 80 to the upper surface of the first information layer 59. Thesecond substrate 60 was placed to the substrate support portion 66a ofthe adhesive-bonding apparatus. Then, the elevation section 63 wasoperated to move the upper-portion support section 61 downwards to bebrought into contact with the spacer 79. While applying a load of 8 kgfrom an upper position to the second substrate 60, it was irradiatedwith light emitted from the light source (the ultraviolet lamp) 81 sothat the resin material 80 was hardened. As a result, the firstseparation layer 62 was formed between the first information layer 59and the second information layer 61. The third substrate 63 was securedto the substrate support section 72 of the adhesive-bonding apparatus.Then, the resin applying nozzle 64 was used to apply the acrylic typeultraviolet curing resin material 80 to the upper surface of the thirdinformation layer 64. The fourth substrate 65 was placed on thesubstrate support section 66a of the adhesive-bonding apparatus. Then,the elevation section 63 was operated so that the upper-portion supportsection 61 was moved downwards to be brought into contact with thespacer 79. While applying a load of 8 kg to the fourth substrate 65 froman upper position, it was irradiated with light emitted from the lightsource (the ultraviolet lamp) 81 so that the resin material 80 washardened. Thus, a second separation layer 67 was formed between thethird information layer 64 and the fourth information layer 66.

Before the adhesive-bonding operation was performed, the thickness ofeach of the inner portion, the intermediate portion and the outerportion was measured to calculate the difference from that after theadhesive-bonding process. Thus, the thicknesses of the first and secondseparation layers 62 and 67 were obtained, thus resulting in that themean thicknesses of the first and second separation layers 62 and 67were 50 μm and 53 μm, respectively. The accuracy at each measurementposition was within ±7 μm. In the foregoing case, the reflectance ofeach of the first and third information layers 59 and 64 at wavelengthof 680 nm was 28.5%, and that of the second and fourth informationlayers 61 and 66 at wavelength of 680 nm was 88.7%. The levels ofeccentricity between the information layers of the first and secondmediums 101 and 103 each having two layers were excellent values, thatis, 30 μm and 28 μm, respectively. Then, the first medium 101 wassecured to the substrate support section 72 of the adhesive-bondingapparatus. The resin applying nozzle 64 was used to apply the acrylictype ultraviolet curring resin material 105 to upper surface of thesecond substrate 60 of the first medium. The second medium 103 wasplaced on the substrate support section 66a of the adhesive-bondingapparatus. Then, the elevation section 63 was operated so that theupper-portion support section 61 was moved downwards to be brought intocontact with spacer 79. While applying a load of 10 kg to the secondmedium 103 from an upper position, it was irradiated with enlarged lightbeam 106 emitted from the light source 81 so that the resin material 105was hardend. Thus the first medium 101 and the second medium 103 areadhesive-bonded to each other via adhesive layer 68.

Information was reproduced from the foregoing recording medium by theoptical system similar to that employed in Example 2 and the servomethod. It was confirmed that a stable focusing operation was performedwhen two information layers were reproduced from either of the surfaceof the first and third substrates 58 and 63. Moreover, focus jumpingbetween the information layers was stably performed. An excellent eyepattern was observed in the reproduced signal from either of theinformation layers. Jitters of each of the first to fourth informationlayers 59, 61, 64 and 66 were measured, thus resulting in excellentstandard deviations with respect to the detection window width of 7.9%,8.3%, 7.9% and 8.2%.

An experiment was performed such that the foregoing recording medium wasallowed to stand in a hot and wet environment in which the temperaturewas 80° C. and the relative humidity was 80%, for 100 houres and thenthe signal was similarly evaluated. As a result, change in the shape wasinhibited, information could stably be reproduced, and the measurementof jitters resulted in a satisfactory value without considerable change.

(EXAMPLE 4)

The specific structure of the optical information recording medium shownin FIG. 18 and recording and reproducing operations with the foregoingmedium will now be described.

The first substrate 31 was made of polycarbonate resin and having, onthe surface thereof, information pits formed in accordance with EFMmodulation to correspond to information signals. The thickness d1 of thefirst substrate 31 was 0.58 mm and the diameter was 120 mm. Theinformation pits formed on the surface of the first substrate 31 werearranged in such a manner that the shortest length of the pit of thepits formed on its surface was 0.44 μm, the pitch depth was 90 nm andthe track pitch was 0.74 μm. A ZnS layer having a thickness of 40 nm wasformed on the surface of the first substrate 31 by the sputtering methodso that the first information layer 32 was formed.

The second substrate 33 was made of polycarbonate resin and had guidegrooves for tracking light beams on the surface thereof. The secondsubstrate 33 had a thickness of 0.58 mm and a diameter of 120 mm. Thepitch of the guide grooves formed on its surface was 1.48 μm, the widthof the groove was the half of the pitch, and the depth was 70 nm. On thesurface of the second substrate 33, there were formed a reflecting layer180 made of Al, a ZnS--SiO₂ dielectric-material layer 181, a Ge--Sb--Terecording thin film layer 182 and a ZnS--SiO₂ dielectric-material layer183 stacked sequentially. Thus, the second information layer 34 wasformed.

The first substrate 31 was secured to the substrate support section 72of the adhesive-bonding apparatus shown in FIG. 6. The resin applyingnozzle 64 was used to apply the ultraviolet curing type resin material80 to the upper surface of the first information layer 32. The secondsubstrate 33 was placed on the substrate support section 66a of theadhesive-bonding apparatus. Then, the elevation section 63 was operatedto downwards move the upper-portion support section 61 to be broughtinto contact with the spacer 79. While applying a load to the secondsubstrate 33 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 35 was formedbetween the first information layer 32 and the second information layer34. The mean thickness of the separation layer 35 was 40 μm with anaccuracy of within ±8 μm or better at each measurement position. Notethat the thickness d1 of the first substrate 31 was 0.6 mm, which wasthe optimum thickness of the base for the objective lens 6 forconverging the light beam 7. The arrangement was determined in such amanner that the optimum point was made to be the central position of theseparation layer 35. In the foregoing case, the reflectance of the firstinformation layer 32 at a wavelength of 680 nm was 10% and thereflectance of the second information layer 34 at a wavelength of 680 nmwas 17%.

Recording and reproducing of information to and from the foregoingrecording medium were evaluated using an optical system comprising alight source for emitting light having a wavelength of 680 nm and anobjective lens having numerical aperture (NA) of 0.6 adaptable to theoptimum base thickness of 0.6 mm, the evaluation being performed at alinear velocity of 6 m/s. Note that the power of the reproducing beamwas 1 mW when the signal was evaluated. As a result, a stable focusingoperation was performed with respect to the first and second informationlayers 32 and 34. Moreover, focus jumping between information layers wasstably performed. As the tracking method, a phase difference methodwhich is suited to the reproduction of a information pits with a narrowtrack pitch, was used for the first information layer 32, and apush-pull method, which is suited to a guiding groove, was used for thesecond information layer 34. An excellent eye pattern was observed inthe reproduced signal from the first information layer 32. Jitters ofeach mark were measured, thus resulting in a standard deviation ofjitters with respect to the detection window width for the code signalof 8.4%.

EFM signals having a shortest mark length of 0.6 μm were recorded onboth land portions and groove portions of the guide grooves on thesecond information layer 34. As a result of irradiation with lightmodulated between recording power of 10 mW and deletion power of 5 mW,excellent eye pattern was observed in each case. The amplitude of thelongest mark 11T was similar to that obtainable from the firstinformation layer 32. Measured jitters resulted in excellent values of9.7% in the land portion and 9.5% in the groove portion. The foregoinginformation signals could be repeatedly rewritten. Note that theforegoing characteristics were equally observed from the inner portionto the outer portion of the substrate.

An experiment was performed such that the foregoing recording medium wasallowed to stand in a hot and wet environment in which the temperaturewas 80° C. and the relative humidity was 80%, for 100 houres and thenthe signal was similarly evaluated. As a result, change in the shape wasinhibited, information could stably be reproduced, and the measurementof jitters resulted in a satisfactory value without considerable change.

As a result, it can be said that the foregoing method is an effectivemethod of manufacturing a recording medium having a plurality ofinformation layers.

(EXAMPLE 5)

The specific structure of the optical information recording medium shownin FIG. 19 and operations for recording and reproducing information toand from the same will now be described.

The first substrate 31 was made of polycarbonate resin and had, on thesurface thereof, information pits formed in accordance with EFMmodulation to correspond to information signals. The thickness d1 of thefirst substrate 31 was 0.58 mm and the diameter of the same was 120 mm.The shortest length of the pit of the information pits on the firstsubstrate 31 was 0.44 μm, the pit depth was 90 nm and the track pitchwas 0.74 μm. On the surface of the first substrate 31, there were formedsequentially by the sputtering method, a dielectric-material layer 194having a thickness of 140 nm and made of ZnS--SiO₂, a dielectric layer195 having a thickness of 30 nm and made of SiO₂ and adielectric-material layer 196 having a thickness of 140 nm and made ofZnS--SiO₂. Thus, the first information layer 32 was formed.

The second substrate 33 was made of polycarbonate resin and had guidegrooves for tracking light beams. The second substrate 33 had athickness of 0.58 mm and a diameter of 120 mm. The pitch of the guidegrooves formed on the surface of the second substrate 33 was 1.1 μm andthe depth of the groove was 50 nm. The second substrate 33 had, on thesurface thereof, a reflecting layer 198 having a thickness of 50 nm andmade of Au, a ZnS--SiO₂ dielectric-material layer 198 having a thicknessof 50 nm, a Ge--Sb--Te recording thin film layer 199 having a thicknessof 10 nm, a ZnS--SiO₂ dielectric-material layer 200 having a thicknessof 20 nm and a semitransparent reflecting layer 201 having a thicknessof 14 nm and made of Au, stacked sequentially. Thus, the secondinformation layer 34 was formed.

The first substrate 31 was secured to the substrate support section 72of the adhesive-bonding apparatus shown in FIG. 6. The resin applyingnozzle 64 was used to apply the ultraviolet curing type resin material80 to the upper surface of the first information layer 32. The secondsubstrate 33 was placed on the substrate support section 66a of theadhesive-bonding apparatus. Then, the elevation section 63 was operatedto downwards move the upper-portion support section 61 to be broughtinto contact with the spacer 79. While applying a load to the secondsubstrate 33 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 35 was formedbetween the first information layer 32 and the second information layer34. The mean thickness of the separation layer 35 was 43 μm with anaccuracy of within 19 μm or better at each measurement position. Notethat the thickness of each substrate was 0.58 mm. The objective lens 6for converging the light beam 7 was adapted to an optimum base thicknessof 0.6 mm. The arrangement was determined in such a manner that theoptimum point was made to be the central position of the separationlayer 35. In the foregoing case, the reflectance of the firstinformation layer 32 at a wavelength of 680 nm was 17% and thereflectance of the second information layer 34 at a wavelength of 680 nmwas 45%.

Recording and reproducing of information to and from the foregoingrecording medium were evaluated by using an optical system comprising alight source for emitting light having a wavelength of 680 nm and anobjective lens having numerical aperture (NA) of 0.6 adaptable to theoptimum base thickness of 0.6 mm, the evaluation being performed at alinear velocity of 1.3 m/s. Note that the power of the reproducing beamwas 1 mW when the signal was evaluated. As a result, a stable focusingoperation was performed with respect to the first and second informationlayers 32 and 34. Moreover, focus jumping between information layers wasstably performed. As the tracking method, a phase difference method wasused for the first information layer 32, and a push-pull method was usedfor the second information layer 34. An excellent eye pattern wasobserved in the reproduced signal from the first information layer 32.Jitters of each mark were measured, thus resulting in a standarddeviation of jitters with respect to the detection window width for thecode signal of 8.1%.

EFM signals having a shortest mark length of 0.6 μm were recorded onboth land portions and groove portions of the guide grooves on thesecond information layer 34. As a result of irradiation with lightmodulated between recording power of 19 mW and deletion power of 9 mW,excellent eye pattern was observed in each case. The amplitude of thelongest mark 11T was similar to that obtainable from the firstinformation layer 32. Measured jitters resulted in 8.3%. The foregoinginformation signals could be repeatedly rewritten. Note that theforegoing characteristics were equally observed from the inner portionto the outer portion of the substrate.

An experiment was performed such that the foregoing recording medium wasallowed to stand in a hot and wet environment in which the temperaturewas 80° C. and the relative humidity was 80%, for 100 houres and thenthe signal was similarly evaluated. As a result, change in the shape wasinhibited, information could stably be reproduced, and the measurementof jitters resulted in a satisfactory value without considerable change.

(EXAMPLE 6)

The specific structure of the optical information recording medium shownin FIG. 20 and operations for recording and reproducing information toand from the same will now be described.

The first substrate 41 was made of polycarbonate resin and having, onthe surface thereof, guide grooves for tracking light beams. Thethickness of the first substrate 41 was 0.58 mm and the diameter of thesame was 120 mm. The pitch of the guide grooves formed on the surface ofthe first substrate 41 was 1.48 μm, the width of the groove was the halfof the track pitch, and the depth of the groove was 50 nm. The firstsubstrate 41 had, on the surface thereof, a ZnS--SiO₂dielectric-material layer 201 having a thickness of 110 nm, a Ge₂ Sb₂Te₅ recording thin film layer 202 having a thickness of 10 nm and aZnS--SiO₂ dielectric-material layer 203 having a thickness of 80 nmstacked sequentially. Thus, a reloadable first information layer 42 wasformed.

The second substrate 43 was made of polycarbonate resin and had, on thesurface thereof, guide grooves for tracking light beams. The thicknessof the second substrate 43 was 0.58 mm and the diameter of the same was120 mm. The pitch of the guide grooves formed on the surface of thesecond substrate 43 was 1.48 μm, the width of the groove was the half ofthe track pitch and the depth of the groove was 50 nm. On the surface ofthe second substrate 43, there were formed a reflecting layer 204 havinga thickness of 100 nm and made of Al, ZnS--SiO₂ dielectric-materiallayer 205 having a thickness of 18 nm, Ge₂ Sb₂ Te₅ recording thin filmlayer 206 having a thickness of 25 nm and ZnS--SiO₂ dielectric-materiallayer 207 having a thickness of 110 nm stacked sequentially. Thus, thesecond information layer 44 was formed.

The first substrate 41 was secured to the substrate support section 72of the adhesive-bonding apparatus shown in FIG. 6. The resin applyingnozzle 64 was operated to apply the ultraviolet curing resin material 80to the upper surface of the first information layer 42. The secondsubstrate 43 was placed on the substrate support section 66a of theadhesive-bonding apparatus. The elevation section 63 was operated tomove the upper-portion support section 61 downwards to be brought intocontact with the spacer 79. While applying a load to the secondsubstrate 43 from an upper position, it was irradiated with lightemitted from the light source (the ultraviolet lamp) 81. Thus, the resinmaterial 80 was hardened so that the separation layer 45 was formedbetween the first information layer 42 and the second information layer43. The mean thickness of the separation layer 45 was 40 μm with anaccuracy of ±7 μm or better at each measurement point. Note that thethickness d1 of the first substrate 41 was adapted to be 0.6 mm, whichwas the optimum base thickness of the objective lens 6 for convergingthe light beam 7. The disposition was determined such that the foregoingoptimum point was made to be the central position of the separationlayer 45. In the non-recording state (a crystal state), the reflectanceof the first information layer 42 was 19%, the transmissivity was 40%and the reflectance of the second information layer 44 was 17%.

Recording and reproducing of information to and from the foregoingrecording medium were evaluated by using an optical system comprising alight source for emitting light having a wavelength of 680 nm and anobjective lens having numerical aperture (NA) of 0.6 adaptable to theoptimum base thickness of 0.6 mm, the evaluation being performed at alinear velocity of 6 m/s. Note that the power of the reproducing beamwas 1 mW when the signal was evaluated. As a result, a stable focusingoperation was performed with respect to the first and second informationlayers 42 and 44. Moreover, focus jumping between information layers wasperformed stably.

EFM signals having a shortest mark length of 0.6 μm were recorded onboth land portions and groove portions of the guide grooves on the firstinformation layer 42. As a result, an excellent eye pattern was observedwhen the recording power was 14 mW. The amplitude of the longest mark11T was similar to that obtainable from the first information layer 42.Measured jitters resulted in excellent values such that it was 10.8% inthe land portion and 11.3% in the groove portion.

EFM signals having a shortest mark length of 0.6 μm were recorded onboth land portions and groove portions of the guide grooves of thesecond information layer 44. An excellent eye pattern was observed ateach portion when the recording power was 18 mW. Measured jittersresulted in 11.7% in the land portion and 12.1% in the groove portionwhich were lower than 13% which was one reference for reproducinginformation. Thus, it was confirmed that information couldsatisfactorily reproduced.

An experiment was performed such that the foregoing recording medium wasallowed to stand in a hot and wet environment in which the temperaturewas 80° C. and the relative humidity was 80%, for 100 houres and thenthe signal was similarly evaluated. As a result, change in the shape wasinhibited, information could stably be reproduced, and the measurementof jitters resulted in a satisfactory value without considerable change.

As a result, it can be said that the method according to the presentinvention is an effective method of manufacturing a recording mediumhaving a plurality of information layers.

What is claimed is:
 1. An optical information recording medium havingtwo information layers and capable of reproducing information signalsfrom said information layers, or recording information signals on saidinformation layers and reproducing the recorded information signals bymeans of irradiation of light beam, said optical information recordingmedium comprising:a first substrate having, on one side thereof, atleast one selected from the group consisting of information pitscorresponding to information signals, guide grooves for tracking andsample pits; a first information layer formed on a surface of said firstsubstrate and having a predetermined transmissivity and a predeterminedreflectance with respect to said light beams; a second substrate having,on one side thereof, at least one selected from the group consisting ofinformation pits corresponding to information signals, guide grooves fortracking and sample pits; a second information layer formed on a secondsubstrate and having a predetermined reflectance; and a separation layerformed between said first information layer and said second informationlayer, said separation layer being transparent with respect to saidlight beam, wherein the thickness of the first substrate issubstantially the same as the thickness of the second substrate.
 2. Theoptical information recording medium according to claim 1, wherein thereflectance of said first information layer is 25% to 40%.
 3. Theoptical information recording medium according to claim 1, wherein saidsecond information layer comprises a reflecting layer, a firstdielectric-material layer, a recording layer and a seconddielectric-material layer, which are sequentially formed on said secondsubstrate.
 4. The optical information recording medium according toclaim 1, wherein said first information layer has a absorption which issubstantially zero with respect to said light beam.
 5. The opticalinformation recording medium according to claim 1, wherein said firstinformation layer comprises at least two layers of thin films.
 6. Theoptical information recording medium according to claim 5, wherein saidfirst information layer comprises a first dielectric-material layer, arecording layer and a second dielectric-material layer, which aresequentially formed on said first substrate.
 7. The optical informationrecording medium according to claim 1, wherein both of said first andsecond substrates have information pits corresponding to informationsignals.
 8. The optical information recording medium according to claim1, wherein said first information layer is a reproduction onlyinformation layer and said second information layer is a recording andreproducing information layer.
 9. The optical information recordingmedium according to claim 1, wherein the thickness of said separationlayer is larger than the focal depth of an optical system for convergingsaid light beam and smaller than a tolerance of a base thickness allowedby said optical system.
 10. The optical information recording mediumaccording to claim 1, wherein information pits or sample pits are formedon said first and second substrates and the shapes of said pits formedon said first substrate are different from the shapes of said pitsformed on said second substrate.
 11. The optical information recordingmedium according to claim 10, wherein the width of said pit formed onsaid second substrate is larger than the width of said pit formed onsaid first substrate.
 12. The optical information recording mediumaccording to claim 1, wherein guide grooves are formed on the surfacesof said first and second substrates and the width of said guide grooveformed on said second substrate is larger than the width of said guidegroove formed on said first substrate.
 13. The optical informationrecording medium according to claim 1, wherein information pits areformed on the surfaces of said first and second substrates and thedensity of said information pits formed on said second substrate perunit area is lower than the density of said information pits formed onsaid first substrate per unit area.
 14. The optical informationrecording medium according to claim 1, wherein sample pits or guidegrooves are formed on the surfaces of said first and second substratesand the pitch of said sample pits or said guide grooves formed on saidsecond substrate is larger than the pitch of said sample pits or saidguide grooves formed on said first substrate.
 15. The opticalinformation recording medium according to claim 1, wherein informationpits or sample pits are formed on the surfaces of the substrates and thedirections of said information pits of said first and second substratesare opposite to each other when viewed from the side from which thelight beam is made incident.
 16. An apparatus for optically recordingand reproducing information which irradiates, with a light beam, anoptical information recording medium having two different informationlayers provided with at least one selected from the group consisting ofinformation pits corresponding to information signals, guide grooves fortracking and sample pits so as to reproduce information signals fromsaid information layers, or record information signals on saidinformation layers and reproduce the recorded information signals, saidapparatus for optically recording and reproducing informationcomprising:an objective lens to converge a light beam emitted from alight source onto said recording medium; a focus control to make thefocal point of said light beam to coincide with one of said informationlayers; a tracking control for controlling the position of said lightbeam to enable said light beam to follow said information pits, guidegrooves or said sample pits; a layer identifier for demodulating asignal for identifying said information layers in accordance with lightreflected by or transmitted through said information pits; a layerselector for selecting an information layer from which said informationsignal is reproduced or on which the information signal is recorded; anda switch means for switching a method of tracking, which is performed bysaid tracking control, to correspond to a result of selection performedby said layer selector.
 17. The apparatus for optically recording andreproducing information according to claim 16, further comprising layercomparison means for subjecting to a comparison a result of selectionperformed by said layer selector and a result of identificationperformed by said layer identifier, and a focus jumping circuit forgenerating a pulse voltage to move the focal point of said light beambetween said information layers to correspond to an output from saidcomparison means.
 18. The apparatus for optically recording andreproducing information according to claim 17, further comprisingtracking polarity inverting means for switching the polarity of saidtracking control in synchronization with the operation of said focusjumping circuit.
 19. The apparatus for optically recording andreproducing information according to claim 16, wherein said focuscontrol comprises first focus control means for making the focal pointof said light beam to coincide with a position near said informationlayer, second focus control means permitted to be operated in a rangesmaller than that of said first focus control means, and focus switchmeans for performing switching to said second focus control means afterthe operation of said first focus control means has been completed. 20.The apparatus for optically recording and reproducing informationaccording to claim 16, wherein said focus control has at least two typesof operation conditions to be adaptable to each of said informationlayers, said focus control being arranged to select one of saidoperation conditions in accordance with a result of selection performedby said layer selector.
 21. The apparatus for optically recording andreproducing information according to claim 16, wherein said trackingcontrol has at least two types of operation conditions to be adaptableto each of said information layers, said tracking control being arrangedto select one of said operation conditions in accordance with a resultof selection performed by said layer selector.
 22. The apparatus foroptically recording and reproducing information according to claim 16,further comprising a photodetector that for performing a forcus controlcomprises a first divisional light receiving surface for receiving aportion of light reflected by said recording medium and a seconddivisional light receiving surface for receiving said reflected light onthe same plane on which said first divisional light receiving surface ispositioned, said second divisional light receiving surface receivingsaid reflected light on the outside of said first divisional lightreceiving surface.
 23. An optical information recording medium havingtwo information layers and for reproducing information signals from saidinformation layers, or recording information signals on said informationlayers and reproducing the recorded information signals by means ofirradiation of light beam, said optical information recording mediumcomprising:a first substrate having, on one side thereof, at least oneselected from the group consisting of information pits corresponding toinformation signals, guide grooves for tracking and sample pits; a firstinformation layer formed on a surface of said first substrate and havinga predetermined transmissivity and a predetermined reflectance withrespect to said light beams; a second substrate having, on one sidethereof, at least one selected from the group consisting of informationpits corresponding to information signals, guide grooves for trackingand sample pits; a second information layer formed on a second substrateand having a predetermined reflectance; and a separation layer formedbetween said first information layer and said second information layer,said separation layer being transparent with respect to said light beam,wherein the following formula is satisfied:

    R1≈1-A1+(2·R2).sup.-1 -{ 1-A1+(2·R2).sup.-1 !.sup.2 -(1-A1).sup.2 }.sup.0.5

wherein R1 is a reflectance of the first information layer with respectto the wavelength of the light beam to reproduce the information, A1 isan absorption ratio of the first information layer, and R2 is areflectance of the second information layer with respect to thewavelength of the light beam.
 24. An optical information recordingmedium comprising:a first substrate having, one side thereof, at leastone selected from the group consisting of information pits correspondingto information signals, guide grooves for tracking light beams andsample pits; a first information layer formed on a surface of said firstsubstrate, made of a material that permits penetration of a portion oflight beams and having a predetermined reflectance; a second substancehaving, on one side thereof, at least one selected from the groupconsisting of information pits corresponding to information signals,guide grooves for tracking light beams and sample pits; a secondinformation layer formed on a surface of said second substrate andhaving a higher reflectance than said first information layer; a thirdsubstrate having, on one side thereof, at least one selected from thegroup consisting of information pits corresponding to informationsignals, guide grooves for tracking light beams and sample pits, saidthird substrate having substantially the same thickness as said firstsubstrate; a third information layer formed on a surface of said thirdsubstrate, made of a material that permits penetration of a portion oflight beams and having a predetermined reflectance; a fourth substratehaving, on one side thereof, at least one selected from the groupconsisting of information pits corresponding to information signals,guide grooves for tracking light beams and sample pits, said fourthsubstrate having substantially the same thickness as said secondsubstrate; a fourth information layer formed on a surface of said fourthsubstrate and having a higher reflectance than said third informationlayer; a first separation layer formed between facing surfaces of saidfirst information layer and said second information layer and comprisingat least one transparent layer; a second separation layer formed betweenfacing surfaces of said third information layer and said fourthinformation layer and having the same thickness as said first separationlayer, wherein said second separation layer comprises at least onetransparent layer; and an adhesive-bonding layer formed between facingsurfaces of said second substrate and said fourth substrate.